diff --git a/eth/downloader/downloader.go b/eth/downloader/downloader.go
index cf8e1becf6..6ac58140a3 100644
--- a/eth/downloader/downloader.go
+++ b/eth/downloader/downloader.go
@@ -153,6 +153,7 @@ type Downloader struct {
 	chainInsertHook  func([]*fetchResult)  // Method to call upon inserting a chain of blocks (possibly in multiple invocations)
 }
 
+// LightChain encapsulates functions required to synchronise a light chain.
 type LightChain interface {
 	// HasHeader verifies a header's presence in the local chain.
 	HasHeader(common.Hash) bool
@@ -173,6 +174,7 @@ type LightChain interface {
 	Rollback([]common.Hash)
 }
 
+// BlockChain encapsulates functions required to sync a (full or fast) blockchain.
 type BlockChain interface {
 	LightChain
 
@@ -281,7 +283,7 @@ func (d *Downloader) RegisterPeer(id string, version int, peer Peer) error {
 	return nil
 }
 
-// RegisterLightPeer injects a light client peer
+// RegisterLightPeer injects a light client peer, wrapping it so it appears as a regular peer.
 func (d *Downloader) RegisterLightPeer(id string, version int, peer LightPeer) error {
 	return d.RegisterPeer(id, version, &lightPeerWrapper{peer})
 }
diff --git a/eth/downloader/peer.go b/eth/downloader/peer.go
index 96870da508..d0dc9a8aa6 100644
--- a/eth/downloader/peer.go
+++ b/eth/downloader/peer.go
@@ -45,7 +45,7 @@ var (
 	errNotRegistered     = errors.New("peer is not registered")
 )
 
-// peer represents an active peer from which hashes and blocks are retrieved.
+// peerConnection represents an active peer from which hashes and blocks are retrieved.
 type peerConnection struct {
 	id string // Unique identifier of the peer
 
@@ -75,12 +75,14 @@ type peerConnection struct {
 	lock    sync.RWMutex
 }
 
+// LightPeer encapsulates the methods required to synchronise with a remote light peer.
 type LightPeer interface {
 	Head() (common.Hash, *big.Int)
 	RequestHeadersByHash(common.Hash, int, int, bool) error
 	RequestHeadersByNumber(uint64, int, int, bool) error
 }
 
+// Peer encapsulates the methods required to synchronise with a remote full peer.
 type Peer interface {
 	LightPeer
 	RequestBodies([]common.Hash) error
@@ -110,7 +112,7 @@ func (w *lightPeerWrapper) RequestNodeData([]common.Hash) error {
 	panic("RequestNodeData not supported in light client mode sync")
 }
 
-// newPeerConnection creates a new downloader peer
+// newPeerConnection creates a new downloader peer.
 func newPeerConnection(id string, version int, peer Peer, logger log.Logger) *peerConnection {
 	return &peerConnection{
 		id:      id,