diff --git a/redo/basicctrls_darwin.m b/redo/basicctrls_darwin.m
index 8e16eb1..6f2769c 100644
--- a/redo/basicctrls_darwin.m
+++ b/redo/basicctrls_darwin.m
@@ -7,6 +7,7 @@
 #define toNSButton(x) ((NSButton *) (x))
 #define toNSTextField(x) ((NSTextField *) (x))
 #define toNSView(x) ((NSView *) (x))
+#define toNSPopover(x) ((NSPopover *) (x))
 #define toNSBox(x) ((NSBox *) (x))
 
 @interface goControlDelegate : NSObject <NSTextFieldDelegate> {
@@ -162,6 +163,54 @@ void textFieldSetText(id t, char *text)
 	[toNSTextField(t) setStringValue:[NSString stringWithUTF8String:text]];
 }
 
+id textfieldOpenInvalidPopover(id textfield, char *reason)
+{
+	// step 1: set up the display
+	NSTextField *label;
+	NSTextAttachmentCell *cell;
+	NSTextAttachment *attachment;
+	NSAttributedString *strImage;
+	NSAttributedString *strText;
+	NSFont *font;
+	NSMutableAttributedString *str;
+
+	// method thanks to Anne in http://stackoverflow.com/a/5303517/3408572
+	// TODO improve appearance
+	label = toNSTextField(newLabel());
+	cell = [[NSTextAttachmentCell alloc] initImageCell:[NSImage imageNamed:NSImageNameCaution]];
+	attachment = [NSTextAttachment new];
+	[attachment setAttachmentCell:cell];
+	strImage = [NSAttributedString attributedStringWithAttachment:attachment];
+	font = [NSFont systemFontOfSize:[NSFont systemFontSizeForControlSize:NSRegularControlSize]];
+	strText = [[NSAttributedString alloc] initWithString:[NSString stringWithUTF8String:reason] attributes:[[font fontDescriptor] fontAttributes]];
+	str = [[NSMutableAttributedString alloc] initWithAttributedString:strImage];
+	[str appendAttributedString:strText];
+	[label setAttributedStringValue:str];
+
+	// step 2: set up the popover
+	NSPopover *popover;
+	NSViewController *vc;
+
+	vc = [NSViewController new];
+	[vc setView:label];
+	popover = [NSPopover new];
+	[popover setContentViewController:vc];
+	[label sizeToFit];
+	[popover setContentSize:[label frame].size];
+
+	// step 3: show the popover
+	// NSMaxYEdge is the bottom edge when looking (maximum edge in window coordinates)
+	[popover showRelativeToRect:NSZeroRect ofView:toNSView(textfield) preferredEdge:NSMaxYEdge];
+
+	return (id) popover;
+}
+
+void textfieldCloseInvalidPopover(id popover)
+{
+	[toNSPopover(popover) close];
+	[toNSPopover(popover) release];
+}
+
 id newLabel(void)
 {
 	NSTextField *l;
diff --git a/redo/objc_darwin.h b/redo/objc_darwin.h
index dca8894..d4a58d0 100644
--- a/redo/objc_darwin.h
+++ b/redo/objc_darwin.h
@@ -71,6 +71,8 @@ extern id newPasswordField(void);
 extern void textfieldSetDelegate(id, void *);
 extern const char *textFieldText(id);
 extern void textFieldSetText(id, char *);
+extern id textfieldOpenInvalidPopover(id, char *);
+extern void textfieldCloseInvalidPopover(id);
 extern id newLabel(void);
 extern id newGroup(id);
 extern const char *groupText(id);
diff --git a/redo/textfield_darwin.go b/redo/textfield_darwin.go
index 1b6ff82..cbe5f8b 100644
--- a/redo/textfield_darwin.go
+++ b/redo/textfield_darwin.go
@@ -12,6 +12,7 @@ import "C"
 type textfield struct {
 	_id		C.id
 	changed	*event
+	invalid	C.id
 }
 
 func finishNewTextField(id C.id) *textfield {
@@ -24,7 +25,7 @@ func finishNewTextField(id C.id) *textfield {
 }
 
 func newTextField() *textfield {
-	return finishNewTextField(C.newTextField()
+	return finishNewTextField(C.newTextField())
 }
 
 func newPasswordField() *textfield {
@@ -46,7 +47,18 @@ func (t *textfield) OnChanged(f func()) {
 }
 
 func (t *textfield) Invalid(reason string) {
-	// TODO
+	// TODO disable animations if reason is still valid
+	// TODO don't steal focus
+	if t.invalid != nil {
+		C.textfieldCloseInvalidPopover(t.invalid)
+		t.invalid = nil
+	}
+	if reason == "" {
+		return
+	}
+	creason := C.CString(reason)
+	defer C.free(unsafe.Pointer(creason))
+	t.invalid = C.textfieldOpenInvalidPopover(t._id, creason)
 }
 
 //export textfieldChanged