diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..1b07dec
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,2 @@
+*.db
+*.dat
diff --git a/README.md b/README.md
new file mode 100644
index 0000000..6eedbb5
--- /dev/null
+++ b/README.md
@@ -0,0 +1,11 @@
+# Blockchain in Go
+
+A blockchain implementation in Go, as described in these articles:
+
+1. [Basic Prototype](https://jeiwan.net/posts/building-blockchain-in-go-part-1/)
+2. [Proof-of-Work](https://jeiwan.net/posts/building-blockchain-in-go-part-2/)
+3. [Persistence and CLI](https://jeiwan.net/posts/building-blockchain-in-go-part-3/)
+4. [Transactions 1](https://jeiwan.net/posts/building-blockchain-in-go-part-4/)
+5. [Addresses](https://jeiwan.net/posts/building-blockchain-in-go-part-5/)
+6. [Transactions 2](https://jeiwan.net/posts/building-blockchain-in-go-part-6/)
+7. [Network](https://jeiwan.net/posts/building-blockchain-in-go-part-7/)
diff --git a/base58.go b/base58.go
new file mode 100644
index 0000000..38c1c35
--- /dev/null
+++ b/base58.go
@@ -0,0 +1,52 @@
+package main
+
+import (
+	"bytes"
+	"math/big"
+)
+
+var b58Alphabet = []byte("123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz")
+
+// Base58Encode encodes a byte array to Base58
+func Base58Encode(input []byte) []byte {
+	var result []byte
+
+	x := big.NewInt(0).SetBytes(input)
+
+	base := big.NewInt(int64(len(b58Alphabet)))
+	zero := big.NewInt(0)
+	mod := &big.Int{}
+
+	for x.Cmp(zero) != 0 {
+		x.DivMod(x, base, mod)
+		result = append(result, b58Alphabet[mod.Int64()])
+	}
+
+	// https://en.bitcoin.it/wiki/Base58Check_encoding#Version_bytes
+	if input[0] == 0x00 {
+		result = append(result, b58Alphabet[0])
+	}
+
+	ReverseBytes(result)
+
+	return result
+}
+
+// Base58Decode decodes Base58-encoded data
+func Base58Decode(input []byte) []byte {
+	result := big.NewInt(0)
+
+	for _, b := range input {
+		charIndex := bytes.IndexByte(b58Alphabet, b)
+		result.Mul(result, big.NewInt(58))
+		result.Add(result, big.NewInt(int64(charIndex)))
+	}
+
+	decoded := result.Bytes()
+
+	if input[0] == b58Alphabet[0] {
+		decoded = append([]byte{0x00}, decoded...)
+	}
+
+	return decoded
+}
diff --git a/base58_test.go b/base58_test.go
new file mode 100644
index 0000000..c65875b
--- /dev/null
+++ b/base58_test.go
@@ -0,0 +1,24 @@
+package main
+
+import (
+	"encoding/hex"
+	"log"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestBase58(t *testing.T) {
+	rawHash := "00010966776006953D5567439E5E39F86A0D273BEED61967F6"
+	hash, err := hex.DecodeString(rawHash)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	encoded := Base58Encode(hash)
+	assert.Equal(t, "16UwLL9Risc3QfPqBUvKofHmBQ7wMtjvM", string(encoded))
+
+	decoded := Base58Decode([]byte("16UwLL9Risc3QfPqBUvKofHmBQ7wMtjvM"))
+	assert.Equal(t, strings.ToLower("00010966776006953D5567439E5E39F86A0D273BEED61967F6"), hex.EncodeToString(decoded))
+}
diff --git a/block.go b/block.go
index dd0afd1..e403e54 100644
--- a/block.go
+++ b/block.go
@@ -2,36 +2,72 @@ package main
 
 import (
 	"bytes"
-	"crypto/sha256"
-	"strconv"
+	"encoding/gob"
+	"log"
 	"time"
 )
 
-// Block keeps block headers
+// Block represents a block in the blockchain
 type Block struct {
 	Timestamp     int64
-	Data          []byte
+	Transactions  []*Transaction
 	PrevBlockHash []byte
 	Hash          []byte
-}
-
-// SetHash calculates and sets block hash
-func (b *Block) SetHash() {
-	timestamp := []byte(strconv.FormatInt(b.Timestamp, 10))
-	headers := bytes.Join([][]byte{b.PrevBlockHash, b.Data, timestamp}, []byte{})
-	hash := sha256.Sum256(headers)
-
-	b.Hash = hash[:]
+	Nonce         int
+	Height        int
 }
 
 // NewBlock creates and returns Block
-func NewBlock(data string, prevBlockHash []byte) *Block {
-	block := &Block{time.Now().Unix(), []byte(data), prevBlockHash, []byte{}}
-	block.SetHash()
+func NewBlock(transactions []*Transaction, prevBlockHash []byte, height int) *Block {
+	block := &Block{time.Now().Unix(), transactions, prevBlockHash, []byte{}, 0, height}
+	pow := NewProofOfWork(block)
+	nonce, hash := pow.Run()
+
+	block.Hash = hash[:]
+	block.Nonce = nonce
+
 	return block
 }
 
 // NewGenesisBlock creates and returns genesis Block
-func NewGenesisBlock() *Block {
-	return NewBlock("Genesis Block", []byte{})
+func NewGenesisBlock(coinbase *Transaction) *Block {
+	return NewBlock([]*Transaction{coinbase}, []byte{}, 0)
+}
+
+// HashTransactions returns a hash of the transactions in the block
+func (b *Block) HashTransactions() []byte {
+	var transactions [][]byte
+
+	for _, tx := range b.Transactions {
+		transactions = append(transactions, tx.Serialize())
+	}
+	mTree := NewMerkleTree(transactions)
+
+	return mTree.RootNode.Data
+}
+
+// Serialize serializes the block
+func (b *Block) Serialize() []byte {
+	var result bytes.Buffer
+	encoder := gob.NewEncoder(&result)
+
+	err := encoder.Encode(b)
+	if err != nil {
+		log.Panic(err)
+	}
+
+	return result.Bytes()
+}
+
+// DeserializeBlock deserializes a block
+func DeserializeBlock(d []byte) *Block {
+	var block Block
+
+	decoder := gob.NewDecoder(bytes.NewReader(d))
+	err := decoder.Decode(&block)
+	if err != nil {
+		log.Panic(err)
+	}
+
+	return &block
 }
diff --git a/blockchain.go b/blockchain.go
index 7d72857..8436035 100644
--- a/blockchain.go
+++ b/blockchain.go
@@ -1,18 +1,360 @@
 package main
 
-// Blockchain keeps a sequence of Blocks
+import (
+	"bytes"
+	"crypto/ecdsa"
+	"encoding/hex"
+	"errors"
+	"fmt"
+	"log"
+	"os"
+
+	"github.com/boltdb/bolt"
+)
+
+const dbFile = "blockchain_%s.db"
+const blocksBucket = "blocks"
+const genesisCoinbaseData = "The Times 03/Jan/2009 Chancellor on brink of second bailout for banks"
+
+// Blockchain implements interactions with a DB
 type Blockchain struct {
-	blocks []*Block
+	tip []byte
+	db  *bolt.DB
 }
 
-// AddBlock saves provided data as a block in the blockchain
-func (bc *Blockchain) AddBlock(data string) {
-	prevBlock := bc.blocks[len(bc.blocks)-1]
-	newBlock := NewBlock(data, prevBlock.Hash)
-	bc.blocks = append(bc.blocks, newBlock)
+// CreateBlockchain creates a new blockchain DB
+func CreateBlockchain(address, nodeID string) *Blockchain {
+	dbFile := fmt.Sprintf(dbFile, nodeID)
+	if dbExists(dbFile) {
+		fmt.Println("Blockchain already exists.")
+		os.Exit(1)
+	}
+
+	var tip []byte
+
+	cbtx := NewCoinbaseTX(address, genesisCoinbaseData)
+	genesis := NewGenesisBlock(cbtx)
+
+	db, err := bolt.Open(dbFile, 0600, nil)
+	if err != nil {
+		log.Panic(err)
+	}
+
+	err = db.Update(func(tx *bolt.Tx) error {
+		b, err := tx.CreateBucket([]byte(blocksBucket))
+		if err != nil {
+			log.Panic(err)
+		}
+
+		err = b.Put(genesis.Hash, genesis.Serialize())
+		if err != nil {
+			log.Panic(err)
+		}
+
+		err = b.Put([]byte("l"), genesis.Hash)
+		if err != nil {
+			log.Panic(err)
+		}
+		tip = genesis.Hash
+
+		return nil
+	})
+	if err != nil {
+		log.Panic(err)
+	}
+
+	bc := Blockchain{tip, db}
+
+	return &bc
 }
 
 // NewBlockchain creates a new Blockchain with genesis Block
-func NewBlockchain() *Blockchain {
-	return &Blockchain{[]*Block{NewGenesisBlock()}}
+func NewBlockchain(nodeID string) *Blockchain {
+	dbFile := fmt.Sprintf(dbFile, nodeID)
+	if dbExists(dbFile) == false {
+		fmt.Println("No existing blockchain found. Create one first.")
+		os.Exit(1)
+	}
+
+	var tip []byte
+	db, err := bolt.Open(dbFile, 0600, nil)
+	if err != nil {
+		log.Panic(err)
+	}
+
+	err = db.Update(func(tx *bolt.Tx) error {
+		b := tx.Bucket([]byte(blocksBucket))
+		tip = b.Get([]byte("l"))
+
+		return nil
+	})
+	if err != nil {
+		log.Panic(err)
+	}
+
+	bc := Blockchain{tip, db}
+
+	return &bc
+}
+
+// AddBlock saves the block into the blockchain
+func (bc *Blockchain) AddBlock(block *Block) {
+	err := bc.db.Update(func(tx *bolt.Tx) error {
+		b := tx.Bucket([]byte(blocksBucket))
+		blockInDb := b.Get(block.Hash)
+
+		if blockInDb != nil {
+			return nil
+		}
+
+		blockData := block.Serialize()
+		err := b.Put(block.Hash, blockData)
+		if err != nil {
+			log.Panic(err)
+		}
+
+		lastHash := b.Get([]byte("l"))
+		lastBlockData := b.Get(lastHash)
+		lastBlock := DeserializeBlock(lastBlockData)
+
+		if block.Height > lastBlock.Height {
+			err = b.Put([]byte("l"), block.Hash)
+			if err != nil {
+				log.Panic(err)
+			}
+			bc.tip = block.Hash
+		}
+
+		return nil
+	})
+	if err != nil {
+		log.Panic(err)
+	}
+}
+
+// FindTransaction finds a transaction by its ID
+func (bc *Blockchain) FindTransaction(ID []byte) (Transaction, error) {
+	bci := bc.Iterator()
+
+	for {
+		block := bci.Next()
+
+		for _, tx := range block.Transactions {
+			if bytes.Compare(tx.ID, ID) == 0 {
+				return *tx, nil
+			}
+		}
+
+		if len(block.PrevBlockHash) == 0 {
+			break
+		}
+	}
+
+	return Transaction{}, errors.New("Transaction is not found")
+}
+
+// FindUTXO finds all unspent transaction outputs and returns transactions with spent outputs removed
+func (bc *Blockchain) FindUTXO() map[string]TXOutputs {
+	UTXO := make(map[string]TXOutputs)
+	spentTXOs := make(map[string][]int)
+	bci := bc.Iterator()
+
+	for {
+		block := bci.Next()
+
+		for _, tx := range block.Transactions {
+			txID := hex.EncodeToString(tx.ID)
+
+		Outputs:
+			for outIdx, out := range tx.Vout {
+				// Was the output spent?
+				if spentTXOs[txID] != nil {
+					for _, spentOutIdx := range spentTXOs[txID] {
+						if spentOutIdx == outIdx {
+							continue Outputs
+						}
+					}
+				}
+
+				outs := UTXO[txID]
+				outs.Outputs = append(outs.Outputs, out)
+				UTXO[txID] = outs
+			}
+
+			if tx.IsCoinbase() == false {
+				for _, in := range tx.Vin {
+					inTxID := hex.EncodeToString(in.Txid)
+					spentTXOs[inTxID] = append(spentTXOs[inTxID], in.Vout)
+				}
+			}
+		}
+
+		if len(block.PrevBlockHash) == 0 {
+			break
+		}
+	}
+
+	return UTXO
+}
+
+// Iterator returns a BlockchainIterat
+func (bc *Blockchain) Iterator() *BlockchainIterator {
+	bci := &BlockchainIterator{bc.tip, bc.db}
+
+	return bci
+}
+
+// GetBestHeight returns the height of the latest block
+func (bc *Blockchain) GetBestHeight() int {
+	var lastBlock Block
+
+	err := bc.db.View(func(tx *bolt.Tx) error {
+		b := tx.Bucket([]byte(blocksBucket))
+		lastHash := b.Get([]byte("l"))
+		blockData := b.Get(lastHash)
+		lastBlock = *DeserializeBlock(blockData)
+
+		return nil
+	})
+	if err != nil {
+		log.Panic(err)
+	}
+
+	return lastBlock.Height
+}
+
+// GetBlock finds a block by its hash and returns it
+func (bc *Blockchain) GetBlock(blockHash []byte) (Block, error) {
+	var block Block
+
+	err := bc.db.View(func(tx *bolt.Tx) error {
+		b := tx.Bucket([]byte(blocksBucket))
+
+		blockData := b.Get(blockHash)
+
+		if blockData == nil {
+			return errors.New("Block is not found.")
+		}
+
+		block = *DeserializeBlock(blockData)
+
+		return nil
+	})
+	if err != nil {
+		return block, err
+	}
+
+	return block, nil
+}
+
+// GetBlockHashes returns a list of hashes of all the blocks in the chain
+func (bc *Blockchain) GetBlockHashes() [][]byte {
+	var blocks [][]byte
+	bci := bc.Iterator()
+
+	for {
+		block := bci.Next()
+
+		blocks = append(blocks, block.Hash)
+
+		if len(block.PrevBlockHash) == 0 {
+			break
+		}
+	}
+
+	return blocks
+}
+
+// MineBlock mines a new block with the provided transactions
+func (bc *Blockchain) MineBlock(transactions []*Transaction) *Block {
+	var lastHash []byte
+	var lastHeight int
+
+	for _, tx := range transactions {
+		// TODO: ignore transaction if it's not valid
+		if bc.VerifyTransaction(tx) != true {
+			log.Panic("ERROR: Invalid transaction")
+		}
+	}
+
+	err := bc.db.View(func(tx *bolt.Tx) error {
+		b := tx.Bucket([]byte(blocksBucket))
+		lastHash = b.Get([]byte("l"))
+
+		blockData := b.Get(lastHash)
+		block := DeserializeBlock(blockData)
+
+		lastHeight = block.Height
+
+		return nil
+	})
+	if err != nil {
+		log.Panic(err)
+	}
+
+	newBlock := NewBlock(transactions, lastHash, lastHeight+1)
+
+	err = bc.db.Update(func(tx *bolt.Tx) error {
+		b := tx.Bucket([]byte(blocksBucket))
+		err := b.Put(newBlock.Hash, newBlock.Serialize())
+		if err != nil {
+			log.Panic(err)
+		}
+
+		err = b.Put([]byte("l"), newBlock.Hash)
+		if err != nil {
+			log.Panic(err)
+		}
+
+		bc.tip = newBlock.Hash
+
+		return nil
+	})
+	if err != nil {
+		log.Panic(err)
+	}
+
+	return newBlock
+}
+
+// SignTransaction signs inputs of a Transaction
+func (bc *Blockchain) SignTransaction(tx *Transaction, privKey ecdsa.PrivateKey) {
+	prevTXs := make(map[string]Transaction)
+
+	for _, vin := range tx.Vin {
+		prevTX, err := bc.FindTransaction(vin.Txid)
+		if err != nil {
+			log.Panic(err)
+		}
+		prevTXs[hex.EncodeToString(prevTX.ID)] = prevTX
+	}
+
+	tx.Sign(privKey, prevTXs)
+}
+
+// VerifyTransaction verifies transaction input signatures
+func (bc *Blockchain) VerifyTransaction(tx *Transaction) bool {
+	if tx.IsCoinbase() {
+		return true
+	}
+
+	prevTXs := make(map[string]Transaction)
+
+	for _, vin := range tx.Vin {
+		prevTX, err := bc.FindTransaction(vin.Txid)
+		if err != nil {
+			log.Panic(err)
+		}
+		prevTXs[hex.EncodeToString(prevTX.ID)] = prevTX
+	}
+
+	return tx.Verify(prevTXs)
+}
+
+func dbExists(dbFile string) bool {
+	if _, err := os.Stat(dbFile); os.IsNotExist(err) {
+		return false
+	}
+
+	return true
 }
diff --git a/blockchain_iterator.go b/blockchain_iterator.go
new file mode 100644
index 0000000..305311d
--- /dev/null
+++ b/blockchain_iterator.go
@@ -0,0 +1,34 @@
+package main
+
+import (
+	"log"
+
+	"github.com/boltdb/bolt"
+)
+
+// BlockchainIterator is used to iterate over blockchain blocks
+type BlockchainIterator struct {
+	currentHash []byte
+	db          *bolt.DB
+}
+
+// Next returns next block starting from the tip
+func (i *BlockchainIterator) Next() *Block {
+	var block *Block
+
+	err := i.db.View(func(tx *bolt.Tx) error {
+		b := tx.Bucket([]byte(blocksBucket))
+		encodedBlock := b.Get(i.currentHash)
+		block = DeserializeBlock(encodedBlock)
+
+		return nil
+	})
+
+	if err != nil {
+		log.Panic(err)
+	}
+
+	i.currentHash = block.PrevBlockHash
+
+	return block
+}
diff --git a/cli.go b/cli.go
new file mode 100644
index 0000000..b597b4c
--- /dev/null
+++ b/cli.go
@@ -0,0 +1,155 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+	"log"
+
+	"os"
+)
+
+// CLI responsible for processing command line arguments
+type CLI struct{}
+
+func (cli *CLI) printUsage() {
+	fmt.Println("Usage:")
+	fmt.Println("  createblockchain -address ADDRESS - Create a blockchain and send genesis block reward to ADDRESS")
+	fmt.Println("  createwallet - Generates a new key-pair and saves it into the wallet file")
+	fmt.Println("  getbalance -address ADDRESS - Get balance of ADDRESS")
+	fmt.Println("  listaddresses - Lists all addresses from the wallet file")
+	fmt.Println("  printchain - Print all the blocks of the blockchain")
+	fmt.Println("  reindexutxo - Rebuilds the UTXO set")
+	fmt.Println("  send -from FROM -to TO -amount AMOUNT -mine - Send AMOUNT of coins from FROM address to TO. Mine on the same node, when -mine is set.")
+	fmt.Println("  startnode -miner ADDRESS - Start a node with ID specified in NODE_ID env. var. -miner enables mining")
+}
+
+func (cli *CLI) validateArgs() {
+	if len(os.Args) < 2 {
+		cli.printUsage()
+		os.Exit(1)
+	}
+}
+
+// Run parses command line arguments and processes commands
+func (cli *CLI) Run() {
+	cli.validateArgs()
+
+	nodeID := os.Getenv("NODE_ID")
+	if nodeID == "" {
+		fmt.Printf("NODE_ID env. var is not set!")
+		os.Exit(1)
+	}
+
+	getBalanceCmd := flag.NewFlagSet("getbalance", flag.ExitOnError)
+	createBlockchainCmd := flag.NewFlagSet("createblockchain", flag.ExitOnError)
+	createWalletCmd := flag.NewFlagSet("createwallet", flag.ExitOnError)
+	listAddressesCmd := flag.NewFlagSet("listaddresses", flag.ExitOnError)
+	printChainCmd := flag.NewFlagSet("printchain", flag.ExitOnError)
+	reindexUTXOCmd := flag.NewFlagSet("reindexutxo", flag.ExitOnError)
+	sendCmd := flag.NewFlagSet("send", flag.ExitOnError)
+	startNodeCmd := flag.NewFlagSet("startnode", flag.ExitOnError)
+
+	getBalanceAddress := getBalanceCmd.String("address", "", "The address to get balance for")
+	createBlockchainAddress := createBlockchainCmd.String("address", "", "The address to send genesis block reward to")
+	sendFrom := sendCmd.String("from", "", "Source wallet address")
+	sendTo := sendCmd.String("to", "", "Destination wallet address")
+	sendAmount := sendCmd.Int("amount", 0, "Amount to send")
+	sendMine := sendCmd.Bool("mine", false, "Mine immediately on the same node")
+	startNodeMiner := startNodeCmd.String("miner", "", "Enable mining mode and send reward to ADDRESS")
+
+	switch os.Args[1] {
+	case "getbalance":
+		err := getBalanceCmd.Parse(os.Args[2:])
+		if err != nil {
+			log.Panic(err)
+		}
+	case "createblockchain":
+		err := createBlockchainCmd.Parse(os.Args[2:])
+		if err != nil {
+			log.Panic(err)
+		}
+	case "createwallet":
+		err := createWalletCmd.Parse(os.Args[2:])
+		if err != nil {
+			log.Panic(err)
+		}
+	case "listaddresses":
+		err := listAddressesCmd.Parse(os.Args[2:])
+		if err != nil {
+			log.Panic(err)
+		}
+	case "printchain":
+		err := printChainCmd.Parse(os.Args[2:])
+		if err != nil {
+			log.Panic(err)
+		}
+	case "reindexutxo":
+		err := reindexUTXOCmd.Parse(os.Args[2:])
+		if err != nil {
+			log.Panic(err)
+		}
+	case "send":
+		err := sendCmd.Parse(os.Args[2:])
+		if err != nil {
+			log.Panic(err)
+		}
+	case "startnode":
+		err := startNodeCmd.Parse(os.Args[2:])
+		if err != nil {
+			log.Panic(err)
+		}
+	default:
+		cli.printUsage()
+		os.Exit(1)
+	}
+
+	if getBalanceCmd.Parsed() {
+		if *getBalanceAddress == "" {
+			getBalanceCmd.Usage()
+			os.Exit(1)
+		}
+		cli.getBalance(*getBalanceAddress, nodeID)
+	}
+
+	if createBlockchainCmd.Parsed() {
+		if *createBlockchainAddress == "" {
+			createBlockchainCmd.Usage()
+			os.Exit(1)
+		}
+		cli.createBlockchain(*createBlockchainAddress, nodeID)
+	}
+
+	if createWalletCmd.Parsed() {
+		cli.createWallet(nodeID)
+	}
+
+	if listAddressesCmd.Parsed() {
+		cli.listAddresses(nodeID)
+	}
+
+	if printChainCmd.Parsed() {
+		cli.printChain(nodeID)
+	}
+
+	if reindexUTXOCmd.Parsed() {
+		cli.reindexUTXO(nodeID)
+	}
+
+	if sendCmd.Parsed() {
+		if *sendFrom == "" || *sendTo == "" || *sendAmount <= 0 {
+			sendCmd.Usage()
+			os.Exit(1)
+		}
+
+		cli.send(*sendFrom, *sendTo, *sendAmount, nodeID, *sendMine)
+	}
+
+	if startNodeCmd.Parsed() {
+		nodeID := os.Getenv("NODE_ID")
+		if nodeID == "" {
+			startNodeCmd.Usage()
+			os.Exit(1)
+		}
+		cli.startNode(nodeID, *startNodeMiner)
+	}
+}
diff --git a/cli_createblockchain.go b/cli_createblockchain.go
new file mode 100644
index 0000000..7e69405
--- /dev/null
+++ b/cli_createblockchain.go
@@ -0,0 +1,19 @@
+package main
+
+import (
+	"fmt"
+	"log"
+)
+
+func (cli *CLI) createBlockchain(address, nodeID string) {
+	if !ValidateAddress(address) {
+		log.Panic("ERROR: Address is not valid")
+	}
+	bc := CreateBlockchain(address, nodeID)
+	defer bc.db.Close()
+
+	UTXOSet := UTXOSet{bc}
+	UTXOSet.Reindex()
+
+	fmt.Println("Done!")
+}
diff --git a/cli_createwallet.go b/cli_createwallet.go
new file mode 100644
index 0000000..0e05f20
--- /dev/null
+++ b/cli_createwallet.go
@@ -0,0 +1,11 @@
+package main
+
+import "fmt"
+
+func (cli *CLI) createWallet(nodeID string) {
+	wallets, _ := NewWallets(nodeID)
+	address := wallets.CreateWallet()
+	wallets.SaveToFile(nodeID)
+
+	fmt.Printf("Your new address: %s\n", address)
+}
diff --git a/cli_getbalance.go b/cli_getbalance.go
new file mode 100644
index 0000000..a86e3dc
--- /dev/null
+++ b/cli_getbalance.go
@@ -0,0 +1,26 @@
+package main
+
+import (
+	"fmt"
+	"log"
+)
+
+func (cli *CLI) getBalance(address, nodeID string) {
+	if !ValidateAddress(address) {
+		log.Panic("ERROR: Address is not valid")
+	}
+	bc := NewBlockchain(nodeID)
+	UTXOSet := UTXOSet{bc}
+	defer bc.db.Close()
+
+	balance := 0
+	pubKeyHash := Base58Decode([]byte(address))
+	pubKeyHash = pubKeyHash[1 : len(pubKeyHash)-4]
+	UTXOs := UTXOSet.FindUTXO(pubKeyHash)
+
+	for _, out := range UTXOs {
+		balance += out.Value
+	}
+
+	fmt.Printf("Balance of '%s': %d\n", address, balance)
+}
diff --git a/cli_listaddress.go b/cli_listaddress.go
new file mode 100644
index 0000000..0d30563
--- /dev/null
+++ b/cli_listaddress.go
@@ -0,0 +1,18 @@
+package main
+
+import (
+	"fmt"
+	"log"
+)
+
+func (cli *CLI) listAddresses(nodeID string) {
+	wallets, err := NewWallets(nodeID)
+	if err != nil {
+		log.Panic(err)
+	}
+	addresses := wallets.GetAddresses()
+
+	for _, address := range addresses {
+		fmt.Println(address)
+	}
+}
diff --git a/cli_printchain.go b/cli_printchain.go
new file mode 100644
index 0000000..81b7edf
--- /dev/null
+++ b/cli_printchain.go
@@ -0,0 +1,31 @@
+package main
+
+import (
+	"fmt"
+	"strconv"
+)
+
+func (cli *CLI) printChain(nodeID string) {
+	bc := NewBlockchain(nodeID)
+	defer bc.db.Close()
+
+	bci := bc.Iterator()
+
+	for {
+		block := bci.Next()
+
+		fmt.Printf("============ Block %x ============\n", block.Hash)
+		fmt.Printf("Height: %d\n", block.Height)
+		fmt.Printf("Prev. block: %x\n", block.PrevBlockHash)
+		pow := NewProofOfWork(block)
+		fmt.Printf("PoW: %s\n\n", strconv.FormatBool(pow.Validate()))
+		for _, tx := range block.Transactions {
+			fmt.Println(tx)
+		}
+		fmt.Printf("\n\n")
+
+		if len(block.PrevBlockHash) == 0 {
+			break
+		}
+	}
+}
diff --git a/cli_reindexutxo.go b/cli_reindexutxo.go
new file mode 100644
index 0000000..87db324
--- /dev/null
+++ b/cli_reindexutxo.go
@@ -0,0 +1,12 @@
+package main
+
+import "fmt"
+
+func (cli *CLI) reindexUTXO(nodeID string) {
+	bc := NewBlockchain(nodeID)
+	UTXOSet := UTXOSet{bc}
+	UTXOSet.Reindex()
+
+	count := UTXOSet.CountTransactions()
+	fmt.Printf("Done! There are %d transactions in the UTXO set.\n", count)
+}
diff --git a/cli_send.go b/cli_send.go
new file mode 100644
index 0000000..75a5930
--- /dev/null
+++ b/cli_send.go
@@ -0,0 +1,39 @@
+package main
+
+import (
+	"fmt"
+	"log"
+)
+
+func (cli *CLI) send(from, to string, amount int, nodeID string, mineNow bool) {
+	if !ValidateAddress(from) {
+		log.Panic("ERROR: Sender address is not valid")
+	}
+	if !ValidateAddress(to) {
+		log.Panic("ERROR: Recipient address is not valid")
+	}
+
+	bc := NewBlockchain(nodeID)
+	UTXOSet := UTXOSet{bc}
+	defer bc.db.Close()
+
+	wallets, err := NewWallets(nodeID)
+	if err != nil {
+		log.Panic(err)
+	}
+	wallet := wallets.GetWallet(from)
+
+	tx := NewUTXOTransaction(&wallet, to, amount, &UTXOSet)
+
+	if mineNow {
+		cbTx := NewCoinbaseTX(from, "")
+		txs := []*Transaction{cbTx, tx}
+
+		newBlock := bc.MineBlock(txs)
+		UTXOSet.Update(newBlock)
+	} else {
+		sendTx(knownNodes[0], tx)
+	}
+
+	fmt.Println("Success!")
+}
diff --git a/cli_startnode.go b/cli_startnode.go
new file mode 100644
index 0000000..d390512
--- /dev/null
+++ b/cli_startnode.go
@@ -0,0 +1,18 @@
+package main
+
+import (
+	"fmt"
+	"log"
+)
+
+func (cli *CLI) startNode(nodeID, minerAddress string) {
+	fmt.Printf("Starting node %s\n", nodeID)
+	if len(minerAddress) > 0 {
+		if ValidateAddress(minerAddress) {
+			fmt.Println("Mining is on. Address to receive rewards: ", minerAddress)
+		} else {
+			log.Panic("Wrong miner address!")
+		}
+	}
+	StartServer(nodeID, minerAddress)
+}
diff --git a/main.go b/main.go
index 68e4642..a48ad8e 100644
--- a/main.go
+++ b/main.go
@@ -1,19 +1,6 @@
 package main
 
-import (
-	"fmt"
-)
-
 func main() {
-	bc := NewBlockchain()
-
-	bc.AddBlock("Send 1 BTC to Ivan")
-	bc.AddBlock("Send 2 more BTC to Ivan")
-
-	for _, block := range bc.blocks {
-		fmt.Printf("Prev. hash: %x\n", block.PrevBlockHash)
-		fmt.Printf("Data: %s\n", block.Data)
-		fmt.Printf("Hash: %x\n", block.Hash)
-		fmt.Println()
-	}
+	cli := CLI{}
+	cli.Run()
 }
diff --git a/merkle_tree.go b/merkle_tree.go
new file mode 100644
index 0000000..7a4156b
--- /dev/null
+++ b/merkle_tree.go
@@ -0,0 +1,65 @@
+package main
+
+import (
+	"crypto/sha256"
+)
+
+// MerkleTree represent a Merkle tree
+type MerkleTree struct {
+	RootNode *MerkleNode
+}
+
+// MerkleNode represent a Merkle tree node
+type MerkleNode struct {
+	Left  *MerkleNode
+	Right *MerkleNode
+	Data  []byte
+}
+
+// NewMerkleTree creates a new Merkle tree from a sequence of data
+func NewMerkleTree(data [][]byte) *MerkleTree {
+	var nodes []MerkleNode
+
+	if len(data)%2 != 0 {
+		data = append(data, data[len(data)-1])
+	}
+
+	for _, datum := range data {
+		node := NewMerkleNode(nil, nil, datum)
+		nodes = append(nodes, *node)
+	}
+
+	for i := 0; i < len(data)/2; i++ {
+		var newLevel []MerkleNode
+
+		for j := 0; j < len(nodes); j += 2 {
+			node := NewMerkleNode(&nodes[j], &nodes[j+1], nil)
+			newLevel = append(newLevel, *node)
+		}
+
+		nodes = newLevel
+	}
+
+	mTree := MerkleTree{&nodes[0]}
+
+	return &mTree
+}
+
+// NewMerkleNode creates a new Merkle tree node
+func NewMerkleNode(left, right *MerkleNode, data []byte) *MerkleNode {
+	mNode := MerkleNode{}
+
+	if left == nil && right == nil {
+		hash := sha256.Sum256(data)
+		mNode.Data = hash[:]
+	} else {
+		prevHashes := append(left.Data, right.Data...)
+		hash := sha256.Sum256(prevHashes)
+		mNode.Data = hash[:]
+	}
+
+	mNode.Left = left
+	mNode.Right = right
+
+	return &mNode
+}
diff --git a/merkle_tree_test.go b/merkle_tree_test.go
new file mode 100644
index 0000000..acff5ff
--- /dev/null
+++ b/merkle_tree_test.go
@@ -0,0 +1,75 @@
+package main
+
+import (
+	"encoding/hex"
+	"fmt"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestNewMerkleNode(t *testing.T) {
+	data := [][]byte{
+		[]byte("node1"),
+		[]byte("node2"),
+		[]byte("node3"),
+	}
+
+	// Level 1
+
+	n1 := NewMerkleNode(nil, nil, data[0])
+	n2 := NewMerkleNode(nil, nil, data[1])
+	n3 := NewMerkleNode(nil, nil, data[2])
+	n4 := NewMerkleNode(nil, nil, data[2])
+
+	// Level 2
+	n5 := NewMerkleNode(n1, n2, nil)
+	n6 := NewMerkleNode(n3, n4, nil)
+
+	// Level 3
+	n7 := NewMerkleNode(n5, n6, nil)
+
+	assert.Equal(
+		t,
+		"64b04b718d8b7c5b6fd17f7ec221945c034cfce3be4118da33244966150c4bd4",
+		hex.EncodeToString(n5.Data),
+		"Level 1 hash 1 is correct",
+	)
+	assert.Equal(
+		t,
+		"08bd0d1426f87a78bfc2f0b13eccdf6f5b58dac6b37a7b9441c1a2fab415d76c",
+		hex.EncodeToString(n6.Data),
+		"Level 1 hash 2 is correct",
+	)
+	assert.Equal(
+		t,
+		"4e3e44e55926330ab6c31892f980f8bfd1a6e910ff1ebc3f778211377f35227e",
+		hex.EncodeToString(n7.Data),
+		"Root hash is correct",
+	)
+}
+
+func TestNewMerkleTree(t *testing.T) {
+	data := [][]byte{
+		[]byte("node1"),
+		[]byte("node2"),
+		[]byte("node3"),
+	}
+	// Level 1
+	n1 := NewMerkleNode(nil, nil, data[0])
+	n2 := NewMerkleNode(nil, nil, data[1])
+	n3 := NewMerkleNode(nil, nil, data[2])
+	n4 := NewMerkleNode(nil, nil, data[2])
+
+	// Level 2
+	n5 := NewMerkleNode(n1, n2, nil)
+	n6 := NewMerkleNode(n3, n4, nil)
+
+	// Level 3
+	n7 := NewMerkleNode(n5, n6, nil)
+
+	rootHash := fmt.Sprintf("%x", n7.Data)
+	mTree := NewMerkleTree(data)
+
+	assert.Equal(t, rootHash, fmt.Sprintf("%x", mTree.RootNode.Data), "Merkle tree root hash is correct")
+}
diff --git a/proofofwork.go b/proofofwork.go
new file mode 100644
index 0000000..cc10c1d
--- /dev/null
+++ b/proofofwork.go
@@ -0,0 +1,86 @@
+package main
+
+import (
+	"bytes"
+	"crypto/sha256"
+	"fmt"
+	"math"
+	"math/big"
+)
+
+var (
+	maxNonce = math.MaxInt64
+)
+
+const targetBits = 16
+
+// ProofOfWork represents a proof-of-work
+type ProofOfWork struct {
+	block  *Block
+	target *big.Int
+}
+
+// NewProofOfWork builds and returns a ProofOfWork
+func NewProofOfWork(b *Block) *ProofOfWork {
+	target := big.NewInt(1)
+	target.Lsh(target, uint(256-targetBits))
+
+	pow := &ProofOfWork{b, target}
+
+	return pow
+}
+
+func (pow *ProofOfWork) prepareData(nonce int) []byte {
+	data := bytes.Join(
+		[][]byte{
+			pow.block.PrevBlockHash,
+			pow.block.HashTransactions(),
+			IntToHex(pow.block.Timestamp),
+			IntToHex(int64(targetBits)),
+			IntToHex(int64(nonce)),
+		},
+		[]byte{},
+	)
+
+	return data
+}
+
+// Run performs a proof-of-work
+func (pow *ProofOfWork) Run() (int, []byte) {
+	var hashInt big.Int
+	var hash [32]byte
+	nonce := 0
+
+	fmt.Printf("Mining a new block")
+	for nonce < maxNonce {
+		data := pow.prepareData(nonce)
+
+		hash = sha256.Sum256(data)
+		if math.Remainder(float64(nonce), 100000) == 0 {
+			fmt.Printf("\r%x", hash)
+		}
+		hashInt.SetBytes(hash[:])
+
+		if hashInt.Cmp(pow.target) == -1 {
+			break
+		} else {
+			nonce++
+		}
+	}
+	fmt.Print("\n\n")
+
+	return nonce, hash[:]
+}
+
+// Validate validates block's PoW
+func (pow *ProofOfWork) Validate() bool {
+	var hashInt big.Int
+
+	data := pow.prepareData(pow.block.Nonce)
+	hash := sha256.Sum256(data)
+	hashInt.SetBytes(hash[:])
+
+	isValid := hashInt.Cmp(pow.target) == -1
+
+	return isValid
+}
diff --git a/server.go b/server.go
new file mode 100644
index 0000000..b13f520
--- /dev/null
+++ b/server.go
@@ -0,0 +1,465 @@
+package main
+
+import (
+	"bytes"
+	"encoding/gob"
+	"encoding/hex"
+	"fmt"
+	"io"
+	"io/ioutil"
+	"log"
+	"net"
+)
+
+const protocol = "tcp"
+const nodeVersion = 1
+const commandLength = 12
+
+var nodeAddress string
+var miningAddress string
+var knownNodes = []string{"localhost:3000"}
+var blocksInTransit = [][]byte{}
+var mempool = make(map[string]Transaction)
+
+type addr struct {
+	AddrList []string
+}
+
+type block struct {
+	AddrFrom string
+	Block    []byte
+}
+
+type getblocks struct {
+	AddrFrom string
+}
+
+type getdata struct {
+	AddrFrom string
+	Type     string
+	ID       []byte
+}
+
+type inv struct {
+	AddrFrom string
+	Type     string
+	Items    [][]byte
+}
+
+type tx struct {
+	AddFrom     string
+	Transaction []byte
+}
+
+type verzion struct {
+	Version    int
+	BestHeight int
+	AddrFrom   string
+}
+
+func commandToBytes(command string) []byte {
+	var bytes [commandLength]byte
+
+	for i, c := range command {
+		bytes[i] = byte(c)
+	}
+
+	return bytes[:]
+}
+
+func bytesToCommand(bytes []byte) string {
+	var command []byte
+
+	for _, b := range bytes {
+		if b != 0x0 {
+			command = append(command, b)
+		}
+	}
+
+	return fmt.Sprintf("%s", command)
+}
+
+func extractCommand(request []byte) []byte {
+	return request[:commandLength]
+}
+
+func requestBlocks() {
+	for _, node := range knownNodes {
+		sendGetBlocks(node)
+	}
+}
+
+func sendAddr(address string) {
+	nodes := addr{knownNodes}
+	nodes.AddrList = append(nodes.AddrList, nodeAddress)
+	payload := gobEncode(nodes)
+	request := append(commandToBytes("addr"), payload...)
+
+	sendData(address, request)
+}
+
+func sendBlock(addr string, b *Block) {
+	data := block{nodeAddress, b.Serialize()}
+	payload := gobEncode(data)
+	request := append(commandToBytes("block"), payload...)
+
+	sendData(addr, request)
+}
+
+func sendData(addr string, data []byte) {
+	conn, err := net.Dial(protocol, addr)
+	if err != nil {
+		fmt.Printf("%s is not available\n", addr)
+		var updatedNodes []string
+
+		for _, node := range knownNodes {
+			if node != addr {
+				updatedNodes = append(updatedNodes, node)
+			}
+		}
+
+		knownNodes = updatedNodes
+
+		return
+	}
+	defer conn.Close()
+
+	_, err = io.Copy(conn, bytes.NewReader(data))
+	if err != nil {
+		log.Panic(err)
+	}
+}
+
+func sendInv(address, kind string, items [][]byte) {
+	inventory := inv{nodeAddress, kind, items}
+	payload := gobEncode(inventory)
+	request := append(commandToBytes("inv"), payload...)
+
+	sendData(address, request)
+}
+
+func sendGetBlocks(address string) {
+	payload := gobEncode(getblocks{nodeAddress})
+	request := append(commandToBytes("getblocks"), payload...)
+
+	sendData(address, request)
+}
+
+func sendGetData(address, kind string, id []byte) {
+	payload := gobEncode(getdata{nodeAddress, kind, id})
+	request := append(commandToBytes("getdata"), payload...)
+
+	sendData(address, request)
+}
+
+func sendTx(addr string, tnx *Transaction) {
+	data := tx{nodeAddress, tnx.Serialize()}
+	payload := gobEncode(data)
+	request := append(commandToBytes("tx"), payload...)
+
+	sendData(addr, request)
+}
+
+func sendVersion(addr string, bc *Blockchain) {
+	bestHeight := bc.GetBestHeight()
+	payload := gobEncode(verzion{nodeVersion, bestHeight, nodeAddress})
+
+	request := append(commandToBytes("version"), payload...)
+
+	sendData(addr, request)
+}
+
+func handleAddr(request []byte) {
+	var buff bytes.Buffer
+	var payload addr
+
+	buff.Write(request[commandLength:])
+	dec := gob.NewDecoder(&buff)
+	err := dec.Decode(&payload)
+	if err != nil {
+		log.Panic(err)
+	}
+
+	knownNodes = append(knownNodes, payload.AddrList...)
+	fmt.Printf("There are %d known nodes now!\n", len(knownNodes))
+	requestBlocks()
+}
+
+func handleBlock(request []byte, bc *Blockchain) {
+	var buff bytes.Buffer
+	var payload block
+
+	buff.Write(request[commandLength:])
+	dec := gob.NewDecoder(&buff)
+	err := dec.Decode(&payload)
+	if err != nil {
+		log.Panic(err)
+	}
+
+	blockData := payload.Block
+	block := DeserializeBlock(blockData)
+
+	fmt.Println("Recevied a new block!")
+	bc.AddBlock(block)
+
+	fmt.Printf("Added block %x\n", block.Hash)
+
+	if len(blocksInTransit) > 0 {
+		blockHash := blocksInTransit[0]
+		sendGetData(payload.AddrFrom, "block", blockHash)
+
+		blocksInTransit = blocksInTransit[1:]
+	} else {
+		UTXOSet := UTXOSet{bc}
+		UTXOSet.Reindex()
+	}
+}
+
+func handleInv(request []byte, bc *Blockchain) {
+	var buff bytes.Buffer
+	var payload inv
+
+	buff.Write(request[commandLength:])
+	dec := gob.NewDecoder(&buff)
+	err := dec.Decode(&payload)
+	if err != nil {
+		log.Panic(err)
+	}
+
+	fmt.Printf("Recevied inventory with %d %s\n", len(payload.Items), payload.Type)
+
+	if payload.Type == "block" {
+		blocksInTransit = payload.Items
+
+		blockHash := payload.Items[0]
+		sendGetData(payload.AddrFrom, "block", blockHash)
+
+		newInTransit := [][]byte{}
+		for _, b := range blocksInTransit {
+			if bytes.Compare(b, blockHash) != 0 {
+				newInTransit = append(newInTransit, b)
+			}
+		}
+		blocksInTransit = newInTransit
+	}
+
+	if payload.Type == "tx" {
+		txID := payload.Items[0]
+
+		if mempool[hex.EncodeToString(txID)].ID == nil {
+			sendGetData(payload.AddrFrom, "tx", txID)
+		}
+	}
+}
+
+func handleGetBlocks(request []byte, bc *Blockchain) {
+	var buff bytes.Buffer
+	var payload getblocks
+
+	buff.Write(request[commandLength:])
+	dec := gob.NewDecoder(&buff)
+	err := dec.Decode(&payload)
+	if err != nil {
+		log.Panic(err)
+	}
+
+	blocks := bc.GetBlockHashes()
+	sendInv(payload.AddrFrom, "block", blocks)
+}
+
+func handleGetData(request []byte, bc *Blockchain) {
+	var buff bytes.Buffer
+	var payload getdata
+
+	buff.Write(request[commandLength:])
+	dec := gob.NewDecoder(&buff)
+	err := dec.Decode(&payload)
+	if err != nil {
+		log.Panic(err)
+	}
+
+	if payload.Type == "block" {
+		block, err := bc.GetBlock([]byte(payload.ID))
+		if err != nil {
+			return
+		}
+
+		sendBlock(payload.AddrFrom, &block)
+	}
+
+	if payload.Type == "tx" {
+		txID := hex.EncodeToString(payload.ID)
+		tx := mempool[txID]
+
+		sendTx(payload.AddrFrom, &tx)
+		// delete(mempool, txID)
+	}
+}
+
+func handleTx(request []byte, bc *Blockchain) {
+	var buff bytes.Buffer
+	var payload tx
+
+	buff.Write(request[commandLength:])
+	dec := gob.NewDecoder(&buff)
+	err := dec.Decode(&payload)
+	if err != nil {
+		log.Panic(err)
+	}
+
+	txData := payload.Transaction
+	tx := DeserializeTransaction(txData)
+	mempool[hex.EncodeToString(tx.ID)] = tx
+
+	if nodeAddress == knownNodes[0] {
+		for _, node := range knownNodes {
+			if node != nodeAddress && node != payload.AddFrom {
+				sendInv(node, "tx", [][]byte{tx.ID})
+			}
+		}
+	} else {
+		if len(mempool) >= 2 && len(miningAddress) > 0 {
+		MineTransactions:
+			var txs []*Transaction
+
+			for id := range mempool {
+				tx := mempool[id]
+				if bc.VerifyTransaction(&tx) {
+					txs = append(txs, &tx)
+				}
+			}
+
+			if len(txs) == 0 {
+				fmt.Println("All transactions are invalid! Waiting for new ones...")
+				return
+			}
+
+			cbTx := NewCoinbaseTX(miningAddress, "")
+			txs = append(txs, cbTx)
+
+			newBlock := bc.MineBlock(txs)
+			UTXOSet := UTXOSet{bc}
+			UTXOSet.Reindex()
+
+			fmt.Println("New block is mined!")
+
+			for _, tx := range txs {
+				txID := hex.EncodeToString(tx.ID)
+				delete(mempool, txID)
+			}
+
+			for _, node := range knownNodes {
+				if node != nodeAddress {
+					sendInv(node, "block", [][]byte{newBlock.Hash})
+				}
+			}
+
+			if len(mempool) > 0 {
+				goto MineTransactions
+			}
+		}
+	}
+}
+
+func handleVersion(request []byte, bc *Blockchain) {
+	var buff bytes.Buffer
+	var payload verzion
+
+	buff.Write(request[commandLength:])
+	dec := gob.NewDecoder(&buff)
+	err := dec.Decode(&payload)
+	if err != nil {
+		log.Panic(err)
+	}
+
+	myBestHeight := bc.GetBestHeight()
+	foreignerBestHeight := payload.BestHeight
+
+	if myBestHeight < foreignerBestHeight {
+		sendGetBlocks(payload.AddrFrom)
+	} else if myBestHeight > foreignerBestHeight {
+		sendVersion(payload.AddrFrom, bc)
+	}
+
+	// sendAddr(payload.AddrFrom)
+	if !nodeIsKnown(payload.AddrFrom) {
+		knownNodes = append(knownNodes, payload.AddrFrom)
+	}
+}
+
+func handleConnection(conn net.Conn, bc *Blockchain) {
+	request, err := ioutil.ReadAll(conn)
+	if err != nil {
+		log.Panic(err)
+	}
+	command := bytesToCommand(request[:commandLength])
+	fmt.Printf("Received %s command\n", command)
+
+	switch command {
+	case "addr":
+		handleAddr(request)
+	case "block":
+		handleBlock(request, bc)
+	case "inv":
+		handleInv(request, bc)
+	case "getblocks":
+		handleGetBlocks(request, bc)
+	case "getdata":
+		handleGetData(request, bc)
+	case "tx":
+		handleTx(request, bc)
+	case "version":
+		handleVersion(request, bc)
+	default:
+		fmt.Println("Unknown command!")
+	}
+
+	conn.Close()
+}
+
+// StartServer starts a node
+func StartServer(nodeID, minerAddress string) {
+	nodeAddress = fmt.Sprintf("localhost:%s", nodeID)
+	miningAddress = minerAddress
+	ln, err := net.Listen(protocol, nodeAddress)
+	if err != nil {
+		log.Panic(err)
+	}
+	defer ln.Close()
+
+	bc := NewBlockchain(nodeID)
+
+	if nodeAddress != knownNodes[0] {
+		sendVersion(knownNodes[0], bc)
+	}
+
+	for {
+		conn, err := ln.Accept()
+		if err != nil {
+			log.Panic(err)
+		}
+		go handleConnection(conn, bc)
+	}
+}
+
+func gobEncode(data interface{}) []byte {
+	var buff bytes.Buffer
+
+	enc := gob.NewEncoder(&buff)
+	err := enc.Encode(data)
+	if err != nil {
+		log.Panic(err)
+	}
+
+	return buff.Bytes()
+}
+
+func nodeIsKnown(addr string) bool {
+	for _, node := range knownNodes {
+		if node == addr {
+			return true
+		}
+	}
+
+	return false
+}
diff --git a/transaction.go b/transaction.go
new file mode 100644
index 0000000..107f015
--- /dev/null
+++ b/transaction.go
@@ -0,0 +1,245 @@
+package main
+
+import (
+	"bytes"
+	"crypto/ecdsa"
+	"crypto/elliptic"
+	"crypto/rand"
+	"crypto/sha256"
+	"math/big"
+	"strings"
+
+	"encoding/gob"
+	"encoding/hex"
+	"fmt"
+	"log"
+)
+
+const subsidy = 10
+
+// Transaction represents a Bitcoin transaction
+type Transaction struct {
+	ID   []byte
+	Vin  []TXInput
+	Vout []TXOutput
+}
+
+// IsCoinbase checks whether the transaction is coinbase
+func (tx Transaction) IsCoinbase() bool {
+	return len(tx.Vin) == 1 && len(tx.Vin[0].Txid) == 0 && tx.Vin[0].Vout == -1
+}
+
+// Serialize returns a serialized Transaction
+func (tx Transaction) Serialize() []byte {
+	var encoded bytes.Buffer
+
+	enc := gob.NewEncoder(&encoded)
+	err := enc.Encode(tx)
+	if err != nil {
+		log.Panic(err)
+	}
+
+	return encoded.Bytes()
+}
+
+// Hash returns the hash of the Transaction
+func (tx *Transaction) Hash() []byte {
+	var hash [32]byte
+
+	txCopy := *tx
+	txCopy.ID = []byte{}
+
+	hash = sha256.Sum256(txCopy.Serialize())
+
+	return hash[:]
+}
+
+// Sign signs each input of a Transaction
+func (tx *Transaction) Sign(privKey ecdsa.PrivateKey, prevTXs map[string]Transaction) {
+	if tx.IsCoinbase() {
+		return
+	}
+
+	for _, vin := range tx.Vin {
+		if prevTXs[hex.EncodeToString(vin.Txid)].ID == nil {
+			log.Panic("ERROR: Previous transaction is not correct")
+		}
+	}
+
+	txCopy := tx.TrimmedCopy()
+
+	for inID, vin := range txCopy.Vin {
+		prevTx := prevTXs[hex.EncodeToString(vin.Txid)]
+		txCopy.Vin[inID].Signature = nil
+		txCopy.Vin[inID].PubKey = prevTx.Vout[vin.Vout].PubKeyHash
+
+		dataToSign := fmt.Sprintf("%x\n", txCopy)
+
+		r, s, err := ecdsa.Sign(rand.Reader, &privKey, []byte(dataToSign))
+		if err != nil {
+			log.Panic(err)
+		}
+		signature := append(r.Bytes(), s.Bytes()...)
+
+		tx.Vin[inID].Signature = signature
+		txCopy.Vin[inID].PubKey = nil
+	}
+}
+
+// String returns a human-readable representation of a transaction
+func (tx Transaction) String() string {
+	var lines []string
+
+	lines = append(lines, fmt.Sprintf("--- Transaction %x:", tx.ID))
+
+	for i, input := range tx.Vin {
+
+		lines = append(lines, fmt.Sprintf("     Input %d:", i))
+		lines = append(lines, fmt.Sprintf("       TXID:      %x", input.Txid))
+		lines = append(lines, fmt.Sprintf("       Out:       %d", input.Vout))
+		lines = append(lines, fmt.Sprintf("       Signature: %x", input.Signature))
+		lines = append(lines, fmt.Sprintf("       PubKey:    %x", input.PubKey))
+	}
+
+	for i, output := range tx.Vout {
+		lines = append(lines, fmt.Sprintf("     Output %d:", i))
+		lines = append(lines, fmt.Sprintf("       Value:  %d", output.Value))
+		lines = append(lines, fmt.Sprintf("       Script: %x", output.PubKeyHash))
+	}
+
+	return strings.Join(lines, "\n")
+}
+
+// TrimmedCopy creates a trimmed copy of Transaction to be used in signing
+func (tx *Transaction) TrimmedCopy() Transaction {
+	var inputs []TXInput
+	var outputs []TXOutput
+
+	for _, vin := range tx.Vin {
+		inputs = append(inputs, TXInput{vin.Txid, vin.Vout, nil, nil})
+	}
+
+	for _, vout := range tx.Vout {
+		outputs = append(outputs, TXOutput{vout.Value, vout.PubKeyHash})
+	}
+
+	txCopy := Transaction{tx.ID, inputs, outputs}
+
+	return txCopy
+}
+
+// Verify verifies signatures of Transaction inputs
+func (tx *Transaction) Verify(prevTXs map[string]Transaction) bool {
+	if tx.IsCoinbase() {
+		return true
+	}
+
+	for _, vin := range tx.Vin {
+		if prevTXs[hex.EncodeToString(vin.Txid)].ID == nil {
+			log.Panic("ERROR: Previous transaction is not correct")
+		}
+	}
+
+	txCopy := tx.TrimmedCopy()
+	curve := elliptic.P256()
+
+	for inID, vin := range tx.Vin {
+		prevTx := prevTXs[hex.EncodeToString(vin.Txid)]
+		txCopy.Vin[inID].Signature = nil
+		txCopy.Vin[inID].PubKey = prevTx.Vout[vin.Vout].PubKeyHash
+
+		r := big.Int{}
+		s := big.Int{}
+		sigLen := len(vin.Signature)
+		r.SetBytes(vin.Signature[:(sigLen / 2)])
+		s.SetBytes(vin.Signature[(sigLen / 2):])
+
+		x := big.Int{}
+		y := big.Int{}
+		keyLen := len(vin.PubKey)
+		x.SetBytes(vin.PubKey[:(keyLen / 2)])
+		y.SetBytes(vin.PubKey[(keyLen / 2):])
+
+		dataToVerify := fmt.Sprintf("%x\n", txCopy)
+
+		rawPubKey := ecdsa.PublicKey{Curve: curve, X: &x, Y: &y}
+		if ecdsa.Verify(&rawPubKey, []byte(dataToVerify), &r, &s) == false {
+			return false
+		}
+		txCopy.Vin[inID].PubKey = nil
+	}
+
+	return true
+}
+
+// NewCoinbaseTX creates a new coinbase transaction
+func NewCoinbaseTX(to, data string) *Transaction {
+	if data == "" {
+		randData := make([]byte, 20)
+		_, err := rand.Read(randData)
+		if err != nil {
+			log.Panic(err)
+		}
+
+		data = fmt.Sprintf("%x", randData)
+	}
+
+	txin := TXInput{[]byte{}, -1, nil, []byte(data)}
+	txout := NewTXOutput(subsidy, to)
+	tx := Transaction{nil, []TXInput{txin}, []TXOutput{*txout}}
+	tx.ID = tx.Hash()
+
+	return &tx
+}
+
+// NewUTXOTransaction creates a new transaction
+func NewUTXOTransaction(wallet *Wallet, to string, amount int, UTXOSet *UTXOSet) *Transaction {
+	var inputs []TXInput
+	var outputs []TXOutput
+
+	pubKeyHash := HashPubKey(wallet.PublicKey)
+	acc, validOutputs := UTXOSet.FindSpendableOutputs(pubKeyHash, amount)
+
+	if acc < amount {
+		log.Panic("ERROR: Not enough funds")
+	}
+
+	// Build a list of inputs
+	for txid, outs := range validOutputs {
+		txID, err := hex.DecodeString(txid)
+		if err != nil {
+			log.Panic(err)
+		}
+
+		for _, out := range outs {
+			input := TXInput{txID, out, nil, wallet.PublicKey}
+			inputs = append(inputs, input)
+		}
+	}
+
+	// Build a list of outputs
+	from := fmt.Sprintf("%s", wallet.GetAddress())
+	outputs = append(outputs, *NewTXOutput(amount, to))
+	if acc > amount {
+		outputs = append(outputs, *NewTXOutput(acc-amount, from)) // a change
+	}
+
+	tx := Transaction{nil, inputs, outputs}
+	tx.ID = tx.Hash()
+	UTXOSet.Blockchain.SignTransaction(&tx, wallet.PrivateKey)
+
+	return &tx
+}
+
+// DeserializeTransaction deserializes a transaction
+func DeserializeTransaction(data []byte) Transaction {
+	var transaction Transaction
+
+	decoder := gob.NewDecoder(bytes.NewReader(data))
+	err := decoder.Decode(&transaction)
+	if err != nil {
+		log.Panic(err)
+	}
+
+	return transaction
+}
diff --git a/transaction_input.go b/transaction_input.go
new file mode 100644
index 0000000..23beeba
--- /dev/null
+++ b/transaction_input.go
@@ -0,0 +1,18 @@
+package main
+
+import "bytes"
+
+// TXInput represents a transaction input
+type TXInput struct {
+	Txid      []byte
+	Vout      int
+	Signature []byte
+	PubKey    []byte
+}
+
+// UsesKey checks whether the address initiated the transaction
+func (in *TXInput) UsesKey(pubKeyHash []byte) bool {
+	lockingHash := HashPubKey(in.PubKey)
+
+	return bytes.Compare(lockingHash, pubKeyHash) == 0
+}
diff --git a/transaction_output.go b/transaction_output.go
new file mode 100644
index 0000000..2ae68de
--- /dev/null
+++ b/transaction_output.go
@@ -0,0 +1,64 @@
+package main
+
+import (
+	"bytes"
+	"encoding/gob"
+	"log"
+)
+
+// TXOutput represents a transaction output
+type TXOutput struct {
+	Value      int
+	PubKeyHash []byte
+}
+
+// Lock signs the output
+func (out *TXOutput) Lock(address []byte) {
+	pubKeyHash := Base58Decode(address)
+	pubKeyHash = pubKeyHash[1 : len(pubKeyHash)-4]
+	out.PubKeyHash = pubKeyHash
+}
+
+// IsLockedWithKey checks if the output can be used by the owner of the pubkey
+func (out *TXOutput) IsLockedWithKey(pubKeyHash []byte) bool {
+	return bytes.Compare(out.PubKeyHash, pubKeyHash) == 0
+}
+
+// NewTXOutput create a new TXOutput
+func NewTXOutput(value int, address string) *TXOutput {
+	txo := &TXOutput{value, nil}
+	txo.Lock([]byte(address))
+
+	return txo
+}
+
+// TXOutputs collects TXOutput
+type TXOutputs struct {
+	Outputs []TXOutput
+}
+
+// Serialize serializes TXOutputs
+func (outs TXOutputs) Serialize() []byte {
+	var buff bytes.Buffer
+
+	enc := gob.NewEncoder(&buff)
+	err := enc.Encode(outs)
+	if err != nil {
+		log.Panic(err)
+	}
+
+	return buff.Bytes()
+}
+
+// DeserializeOutputs deserializes TXOutputs
+func DeserializeOutputs(data []byte) TXOutputs {
+	var outputs TXOutputs
+
+	dec := gob.NewDecoder(bytes.NewReader(data))
+	err := dec.Decode(&outputs)
+	if err != nil {
+		log.Panic(err)
+	}
+
+	return outputs
+}
diff --git a/utils.go b/utils.go
new file mode 100644
index 0000000..aecf919
--- /dev/null
+++ b/utils.go
@@ -0,0 +1,25 @@
+package main
+
+import (
+	"bytes"
+	"encoding/binary"
+	"log"
+)
+
+// IntToHex converts an int64 to a byte array
+func IntToHex(num int64) []byte {
+	buff := new(bytes.Buffer)
+	err := binary.Write(buff, binary.BigEndian, num)
+	if err != nil {
+		log.Panic(err)
+	}
+
+	return buff.Bytes()
+}
+
+// ReverseBytes reverses a byte array
+func ReverseBytes(data []byte) {
+	for i, j := 0, len(data)-1; i < j; i, j = i+1, j-1 {
+		data[i], data[j] = data[j], data[i]
+	}
+}
diff --git a/utxo_set.go b/utxo_set.go
new file mode 100644
index 0000000..180ce04
--- /dev/null
+++ b/utxo_set.go
@@ -0,0 +1,193 @@
+package main
+
+import (
+	"encoding/hex"
+	"log"
+
+	"github.com/boltdb/bolt"
+)
+
+const utxoBucket = "chainstate"
+
+// UTXOSet represents UTXO set
+type UTXOSet struct {
+	Blockchain *Blockchain
+}
+
+// FindSpendableOutputs finds and returns unspent outputs to reference in inputs
+func (u UTXOSet) FindSpendableOutputs(pubkeyHash []byte, amount int) (int, map[string][]int) {
+	unspentOutputs := make(map[string][]int)
+	accumulated := 0
+	db := u.Blockchain.db
+
+	err := db.View(func(tx *bolt.Tx) error {
+		b := tx.Bucket([]byte(utxoBucket))
+		c := b.Cursor()
+
+		for k, v := c.First(); k != nil; k, v = c.Next() {
+			txID := hex.EncodeToString(k)
+			outs := DeserializeOutputs(v)
+
+			for outIdx, out := range outs.Outputs {
+				if out.IsLockedWithKey(pubkeyHash) && accumulated < amount {
+					accumulated += out.Value
+					unspentOutputs[txID] = append(unspentOutputs[txID], outIdx)
+				}
+			}
+		}
+
+		return nil
+	})
+	if err != nil {
+		log.Panic(err)
+	}
+
+	return accumulated, unspentOutputs
+}
+
+// FindUTXO finds UTXO for a public key hash
+func (u UTXOSet) FindUTXO(pubKeyHash []byte) []TXOutput {
+	var UTXOs []TXOutput
+	db := u.Blockchain.db
+
+	err := db.View(func(tx *bolt.Tx) error {
+		b := tx.Bucket([]byte(utxoBucket))
+		c := b.Cursor()
+
+		for k, v := c.First(); k != nil; k, v = c.Next() {
+			outs := DeserializeOutputs(v)
+
+			for _, out := range outs.Outputs {
+				if out.IsLockedWithKey(pubKeyHash) {
+					UTXOs = append(UTXOs, out)
+				}
+			}
+		}
+
+		return nil
+	})
+	if err != nil {
+		log.Panic(err)
+	}
+
+	return UTXOs
+}
+
+// CountTransactions returns the number of transactions in the UTXO set
+func (u UTXOSet) CountTransactions() int {
+	db := u.Blockchain.db
+	counter := 0
+
+	err := db.View(func(tx *bolt.Tx) error {
+		b := tx.Bucket([]byte(utxoBucket))
+		c := b.Cursor()
+
+		for k, _ := c.First(); k != nil; k, _ = c.Next() {
+			counter++
+		}
+
+		return nil
+	})
+	if err != nil {
+		log.Panic(err)
+	}
+
+	return counter
+}
+
+// Reindex rebuilds the UTXO set
+func (u UTXOSet) Reindex() {
+	db := u.Blockchain.db
+	bucketName := []byte(utxoBucket)
+
+	err := db.Update(func(tx *bolt.Tx) error {
+		err := tx.DeleteBucket(bucketName)
+		if err != nil && err != bolt.ErrBucketNotFound {
+			log.Panic(err)
+		}
+
+		_, err = tx.CreateBucket(bucketName)
+		if err != nil {
+			log.Panic(err)
+		}
+
+		return nil
+	})
+	if err != nil {
+		log.Panic(err)
+	}
+
+	UTXO := u.Blockchain.FindUTXO()
+
+	err = db.Update(func(tx *bolt.Tx) error {
+		b := tx.Bucket(bucketName)
+
+		for txID, outs := range UTXO {
+			key, err := hex.DecodeString(txID)
+			if err != nil {
+				log.Panic(err)
+			}
+
+			err = b.Put(key, outs.Serialize())
+			if err != nil {
+				log.Panic(err)
+			}
+		}
+
+		return nil
+	})
+}
+
+// Update updates the UTXO set with transactions from the Block
+// The Block is considered to be the tip of a blockchain
+func (u UTXOSet) Update(block *Block) {
+	db := u.Blockchain.db
+
+	err := db.Update(func(tx *bolt.Tx) error {
+		b := tx.Bucket([]byte(utxoBucket))
+
+		for _, tx := range block.Transactions {
+			if tx.IsCoinbase() == false {
+				for _, vin := range tx.Vin {
+					updatedOuts := TXOutputs{}
+					outsBytes := b.Get(vin.Txid)
+					outs := DeserializeOutputs(outsBytes)
+
+					for outIdx, out := range outs.Outputs {
+						if outIdx != vin.Vout {
+							updatedOuts.Outputs = append(updatedOuts.Outputs, out)
+						}
+					}
+
+					if len(updatedOuts.Outputs) == 0 {
+						err := b.Delete(vin.Txid)
+						if err != nil {
+							log.Panic(err)
+						}
+					} else {
+						err := b.Put(vin.Txid, updatedOuts.Serialize())
+						if err != nil {
+							log.Panic(err)
+						}
+					}
+
+				}
+			}
+
+			newOutputs := TXOutputs{}
+			for _, out := range tx.Vout {
+				newOutputs.Outputs = append(newOutputs.Outputs, out)
+			}
+
+			err := b.Put(tx.ID, newOutputs.Serialize())
+			if err != nil {
+				log.Panic(err)
+			}
+		}
+
+		return nil
+	})
+	if err != nil {
+		log.Panic(err)
+	}
+}
diff --git a/wallet.go b/wallet.go
new file mode 100644
index 0000000..506b544
--- /dev/null
+++ b/wallet.go
@@ -0,0 +1,86 @@
+package main
+
+import (
+	"bytes"
+	"crypto/ecdsa"
+	"crypto/elliptic"
+	"crypto/rand"
+	"crypto/sha256"
+	"log"
+
+	"golang.org/x/crypto/ripemd160"
+)
+
+const version = byte(0x00)
+const addressChecksumLen = 4
+
+// Wallet stores private and public keys
+type Wallet struct {
+	PrivateKey ecdsa.PrivateKey
+	PublicKey  []byte
+}
+
+// NewWallet creates and returns a Wallet
+func NewWallet() *Wallet {
+	private, public := newKeyPair()
+	wallet := Wallet{private, public}
+
+	return &wallet
+}
+
+// GetAddress returns wallet address
+func (w Wallet) GetAddress() []byte {
+	pubKeyHash := HashPubKey(w.PublicKey)
+
+	versionedPayload := append([]byte{version}, pubKeyHash...)
+	checksum := checksum(versionedPayload)
+
+	fullPayload := append(versionedPayload, checksum...)
+	address := Base58Encode(fullPayload)
+
+	return address
+}
+
+// HashPubKey hashes public key
+func HashPubKey(pubKey []byte) []byte {
+	publicSHA256 := sha256.Sum256(pubKey)
+
+	RIPEMD160Hasher := ripemd160.New()
+	_, err := RIPEMD160Hasher.Write(publicSHA256[:])
+	if err != nil {
+		log.Panic(err)
+	}
+	publicRIPEMD160 := RIPEMD160Hasher.Sum(nil)
+
+	return publicRIPEMD160
+}
+
+// ValidateAddress check if address if valid
+func ValidateAddress(address string) bool {
+	pubKeyHash := Base58Decode([]byte(address))
+	actualChecksum := pubKeyHash[len(pubKeyHash)-addressChecksumLen:]
+	version := pubKeyHash[0]
+	pubKeyHash = pubKeyHash[1 : len(pubKeyHash)-addressChecksumLen]
+	targetChecksum := checksum(append([]byte{version}, pubKeyHash...))
+
+	return bytes.Compare(actualChecksum, targetChecksum) == 0
+}
+
+// Checksum generates a checksum for a public key
+func checksum(payload []byte) []byte {
+	firstSHA := sha256.Sum256(payload)
+	secondSHA := sha256.Sum256(firstSHA[:])
+
+	return secondSHA[:addressChecksumLen]
+}
+
+func newKeyPair() (ecdsa.PrivateKey, []byte) {
+	curve := elliptic.P256()
+	private, err := ecdsa.GenerateKey(curve, rand.Reader)
+	if err != nil {
+		log.Panic(err)
+	}
+	pubKey := append(private.PublicKey.X.Bytes(), private.PublicKey.Y.Bytes()...)
+
+	return *private, pubKey
+}
diff --git a/wallets.go b/wallets.go
new file mode 100644
index 0000000..9f376f5
--- /dev/null
+++ b/wallets.go
@@ -0,0 +1,98 @@
+package main
+
+import (
+	"bytes"
+	"crypto/elliptic"
+	"encoding/gob"
+	"fmt"
+	"io/ioutil"
+	"log"
+	"os"
+)
+
+const walletFile = "wallet_%s.dat"
+
+// Wallets stores a collection of wallets
+type Wallets struct {
+	Wallets map[string]*Wallet
+}
+
+// NewWallets creates Wallets and fills it from a file if it exists
+func NewWallets(nodeID string) (*Wallets, error) {
+	wallets := Wallets{}
+	wallets.Wallets = make(map[string]*Wallet)
+
+	err := wallets.LoadFromFile(nodeID)
+
+	return &wallets, err
+}
+
+// CreateWallet adds a Wallet to Wallets
+func (ws *Wallets) CreateWallet() string {
+	wallet := NewWallet()
+	address := fmt.Sprintf("%s", wallet.GetAddress())
+
+	ws.Wallets[address] = wallet
+
+	return address
+}
+
+// GetAddresses returns an array of addresses stored in the wallet file
+func (ws *Wallets) GetAddresses() []string {
+	var addresses []string
+
+	for address := range ws.Wallets {
+		addresses = append(addresses, address)
+	}
+
+	return addresses
+}
+
+// GetWallet returns a Wallet by its address
+func (ws Wallets) GetWallet(address string) Wallet {
+	return *ws.Wallets[address]
+}
+
+// LoadFromFile loads wallets from the file
+func (ws *Wallets) LoadFromFile(nodeID string) error {
+	walletFile := fmt.Sprintf(walletFile, nodeID)
+	if _, err := os.Stat(walletFile); os.IsNotExist(err) {
+		return err
+	}
+
+	fileContent, err := ioutil.ReadFile(walletFile)
+	if err != nil {
+		log.Panic(err)
+	}
+
+	var wallets Wallets
+	gob.Register(elliptic.P256())
+	decoder := gob.NewDecoder(bytes.NewReader(fileContent))
+	err = decoder.Decode(&wallets)
+	if err != nil {
+		log.Panic(err)
+	}
+
+	ws.Wallets = wallets.Wallets
+
+	return nil
+}
+
+// SaveToFile saves wallets to a file
+func (ws Wallets) SaveToFile(nodeID string) {
+	var content bytes.Buffer
+	walletFile := fmt.Sprintf(walletFile, nodeID)
+
+	gob.Register(elliptic.P256())
+
+	encoder := gob.NewEncoder(&content)
+	err := encoder.Encode(ws)
+	if err != nil {
+		log.Panic(err)
+	}
+
+	err = ioutil.WriteFile(walletFile, content.Bytes(), 0644)
+	if err != nil {
+		log.Panic(err)
+	}
+}