diff --git a/TODO b/TODO
index 40552c4..efd1a81 100644
--- a/TODO
+++ b/TODO
@@ -4,7 +4,6 @@ X Open DB.
 X Initialize transaction.
 - Cursor First, Goto(key), Next
 - RWTransaction.insert()
-  - page split
   - rebalance
   - adjust cursors
 - RWTransaction Commmit
diff --git a/bnode.go b/bnode.go
index 9fef154..3ad4f58 100644
--- a/bnode.go
+++ b/bnode.go
@@ -15,5 +15,6 @@ type bnode struct {
 
 // key returns a byte slice of the node key.
 func (n *bnode) key() []byte {
-	return (*[MaxKeySize]byte)(unsafe.Pointer(&n))[n.pos : n.pos+n.ksize]
+	buf := (*[maxAllocSize]byte)(unsafe.Pointer(n))
+	return buf[n.pos : n.pos+n.ksize]
 }
diff --git a/branch.go b/branch.go
index c9fc7ca..7cba9d4 100644
--- a/branch.go
+++ b/branch.go
@@ -11,6 +11,15 @@ type branch struct {
 	items branchItems
 }
 
+// size returns the size of the branch after serialization.
+func (b *branch) size() int {
+	var size int = pageHeaderSize
+	for _, item := range b.items {
+		size += bnodeSize + len(item.key)
+	}
+	return size
+}
+
 // put adds a new node or replaces an existing node.
 func (b *branch) put(id pgid, newid pgid, key []byte, replace bool) {
 	var index int
@@ -40,10 +49,65 @@ func (b *branch) read(page *page) {
 	for i := 0; i < ncount; i++ {
 		bnode := &bnodes[i]
 		item := &b.items[i]
+		item.pgid = bnode.pgid
 		item.key = bnode.key()
 	}
 }
 
+// write writes the items onto a branch page.
+func (b *branch) write(p *page) {
+	// Initialize page.
+	p.flags |= p_branch
+	p.count = uint16(len(b.items))
+
+	// Loop over each item and write it to the page.
+	bnodes := (*[maxNodesPerPage]bnode)(unsafe.Pointer(&p.ptr))
+	buf := (*[maxAllocSize]byte)(unsafe.Pointer(&p.ptr))[lnodeSize*len(b.items):]
+	for index, item := range b.items {
+		// Write node.
+		bnode := &bnodes[index]
+		bnode.pgid = item.pgid
+		bnode.pos = uint32(uintptr(unsafe.Pointer(&buf[0])) - uintptr(unsafe.Pointer(bnode)))
+		bnode.ksize = uint32(len(item.key))
+
+		// Write key to the end of the page.
+		copy(buf[0:], item.key)
+		buf = buf[len(item.key):]
+	}
+}
+
+// split divides up the noes in the branch into appropriately sized groups.
+func (b *branch) split(pageSize int) []*branch {
+	// Ignore the split if the page doesn't have at least enough nodes for
+	// multiple pages or if the data can fit on a single page.
+	if len(b.items) <= (minKeysPerPage * 2) || b.size() < pageSize {
+		return []*branch{b}
+	}
+
+	// Set fill threshold to 50%.
+	threshold := pageSize / 2
+
+	// Otherwise group into smaller pages and target a given fill size.
+	size := 0
+	current := &branch{}
+	branches := make([]*branch, 0)
+
+	for index, item := range b.items {
+		nodeSize := bnodeSize + len(item.key)
+
+		if (len(current.items) >= minKeysPerPage && index < len(b.items)-minKeysPerPage && size+nodeSize > threshold) {
+			size = pageHeaderSize
+			branches = append(branches, current)
+			current = &branch{}
+		}
+
+		size += nodeSize
+		current.items = append(current.items, item)
+	}
+	branches = append(branches, current)
+
+	return branches
+}
 
 type branchItems []branchItem
 
diff --git a/branch_test.go b/branch_test.go
index 0687b20..4a810c8 100644
--- a/branch_test.go
+++ b/branch_test.go
@@ -2,6 +2,7 @@ package bolt
 
 import (
 	"testing"
+	"unsafe"
 
 	"github.com/stretchr/testify/assert"
 )
@@ -46,3 +47,115 @@ func TestBranchPutInsert(t *testing.T) {
 	assert.Equal(t, b.items[3].pgid, pgid(5))
 	assert.Equal(t, string(b.items[3].key), "zzz")
 }
+
+// Ensure that a branch can deserialize from a page.
+func TestBranchRead(t *testing.T) {
+	// Create a page.
+	var buf [4096]byte
+	page := (*page)(unsafe.Pointer(&buf[0]))
+	page.count = 2
+
+	// Insert 2 items at the beginning. sizeof(bnode) == 16
+	nodes := (*[3]bnode)(unsafe.Pointer(&page.ptr))
+	nodes[0] = bnode{pos: 32, ksize: 3, pgid: 100}  // pos = sizeof(bnode) * 2
+	nodes[1] = bnode{pos: 19, ksize: 10, pgid: 101} // pos = sizeof(bnode) + 3
+
+	// Write data for the nodes at the end.
+	data := (*[4096]byte)(unsafe.Pointer(&nodes[2]))
+	copy(data[:], []byte("bar"))
+	copy(data[3:], []byte("helloworld"))
+
+	// Deserialize page into a branch.
+	b := &branch{}
+	b.read(page)
+
+	// Check that there are two items with correct data.
+	assert.Equal(t, len(b.items), 2)
+	assert.Equal(t, b.items[0].key, []byte("bar"))
+	assert.Equal(t, b.items[1].key, []byte("helloworld"))
+}
+
+// Ensure that a branch can serialize itself.
+func TestBranchWrite(t *testing.T) {
+	b := &branch{
+		items: branchItems{
+			branchItem{pgid: 1, key: []byte("susy")},
+			branchItem{pgid: 2, key: []byte("ricki")},
+			branchItem{pgid: 3, key: []byte("john")},
+		},
+	}
+
+	// Write it to a page.
+	var buf [4096]byte
+	p := (*page)(unsafe.Pointer(&buf[0]))
+	b.write(p)
+
+	// Read the page back in.
+	b2 := &branch{}
+	b2.read(p)
+
+	// Check that the two pages are the same.
+	assert.Equal(t, len(b2.items), 3)
+	assert.Equal(t, b2.items[0].pgid, pgid(1))
+	assert.Equal(t, b2.items[0].key, []byte("susy"))
+	assert.Equal(t, b2.items[1].pgid, pgid(2))
+	assert.Equal(t, b2.items[1].key, []byte("ricki"))
+	assert.Equal(t, b2.items[2].pgid, pgid(3))
+	assert.Equal(t, b2.items[2].key, []byte("john"))
+}
+
+// Ensure that a branch can split into appropriate subgroups.
+func TestBranchSplit(t *testing.T) {
+	// Create a branch.
+	b := &branch{
+		items: branchItems{
+			branchItem{pgid: 1, key: []byte("00000001")},
+			branchItem{pgid: 2, key: []byte("00000002")},
+			branchItem{pgid: 3, key: []byte("00000003")},
+			branchItem{pgid: 4, key: []byte("00000004")},
+			branchItem{pgid: 5, key: []byte("00000005")},
+		},
+	}
+
+	// Split between 3 & 4.
+	branches := b.split(100)
+
+	assert.Equal(t, len(branches), 2)
+	assert.Equal(t, len(branches[0].items), 2)
+	assert.Equal(t, len(branches[1].items), 3)
+}
+
+// Ensure that a branch with the minimum number of items just returns a single branch.
+func TestBranchSplitWithMinKeys(t *testing.T) {
+	// Create a branch.
+	b := &branch{
+		items: branchItems{
+			branchItem{pgid: 1, key: []byte("00000001")},
+			branchItem{pgid: 2, key: []byte("00000002")},
+		},
+	}
+
+	// Split.
+	branches := b.split(20)
+	assert.Equal(t, len(branches), 1)
+	assert.Equal(t, len(branches[0].items), 2)
+}
+
+// Ensure that a branch that has keys that all fit on a page just returns one branch.
+func TestBranchSplitFitsInPage(t *testing.T) {
+	// Create a branch.
+	b := &branch{
+		items: branchItems{
+			branchItem{pgid: 1, key: []byte("00000001")},
+			branchItem{pgid: 2, key: []byte("00000002")},
+			branchItem{pgid: 3, key: []byte("00000003")},
+			branchItem{pgid: 4, key: []byte("00000004")},
+			branchItem{pgid: 5, key: []byte("00000005")},
+		},
+	}
+
+	// Split.
+	branches := b.split(4096)
+	assert.Equal(t, len(branches), 1)
+	assert.Equal(t, len(branches[0].items), 5)
+}
diff --git a/leaf.go b/leaf.go
index dcd343f..be5d447 100644
--- a/leaf.go
+++ b/leaf.go
@@ -6,13 +6,21 @@ import (
 	"unsafe"
 )
 
-// leaf represents a temporary in-memory leaf page.
-// It is deserialized from an memory-mapped page and is not restricted by page size.
+// leaf represents an in-memory, deserialized leaf page.
 type leaf struct {
 	parent *branch
 	items leafItems
 }
 
+// size returns the size of the leaf after serialization.
+func (l *leaf) size() int {
+	var size int = pageHeaderSize
+	for _, item := range l.items {
+		size += lnodeSize + len(item.key) + len(item.value)
+	}
+	return size
+}
+
 // put inserts or replaces a key on a leaf page.
 func (l *leaf) put(key []byte, value []byte) {
 	// Find insertion index.
@@ -29,15 +37,6 @@ func (l *leaf) put(key []byte, value []byte) {
 	l.items[index].value = value
 }
 
-// size returns the size of the leaf after serialization.
-func (l *leaf) size() int {
-	var size int = pageHeaderSize
-	for _, item := range l.items {
-		size += lnodeSize + len(item.key) + len(item.value)
-	}
-	return size
-}
-
 // read initializes the item data from an on-disk page.
 func (l *leaf) read(p *page) {
 	ncount := int(p.count)
@@ -83,8 +82,8 @@ func (l *leaf) split(pageSize int) []*leaf {
 		return []*leaf{l}
 	}
 
-	// Set fill threshold to 25%.
-	threshold := pageSize >> 4
+	// Set fill threshold to 50%.
+	threshold := pageSize / 2
 
 	// Otherwise group into smaller pages and target a given fill size.
 	size := 0
diff --git a/leaf_test.go b/leaf_test.go
index 3b2235f..ce98e37 100644
--- a/leaf_test.go
+++ b/leaf_test.go
@@ -7,7 +7,7 @@ import (
 	"github.com/stretchr/testify/assert"
 )
 
-// Ensure that a temporary page can insert a key/value.
+// Ensure that a leaf can insert a key/value.
 func TestLeafPut(t *testing.T) {
 	l := &leaf{items: make(leafItems, 0)}
 	l.put([]byte("baz"), []byte("2"))
@@ -20,7 +20,7 @@ func TestLeafPut(t *testing.T) {
 	assert.Equal(t, l.items[2], leafItem{[]byte("foo"), []byte("3")})
 }
 
-// Ensure that a temporary page can deserialize from a page.
+// Ensure that a leaf can deserialize from a page.
 func TestLeafRead(t *testing.T) {
 	// Create a page.
 	var buf [4096]byte
@@ -37,7 +37,7 @@ func TestLeafRead(t *testing.T) {
 	copy(data[:], []byte("barfooz"))
 	copy(data[7:], []byte("helloworldbye"))
 
-	// Deserialize page into a temporary page.
+	// Deserialize page into a leaf.
 	l := &leaf{}
 	l.read(page)
 
@@ -49,9 +49,9 @@ func TestLeafRead(t *testing.T) {
 	assert.Equal(t, l.items[1].value, []byte("bye"))
 }
 
-// Ensure that a temporary page can serialize itself.
+// Ensure that a leaf can serialize itself.
 func TestLeafWrite(t *testing.T) {
-	// Create a temp page.
+	// Create a leaf.
 	l := &leaf{items: make(leafItems, 0)}
 	l.put([]byte("susy"), []byte("que"))
 	l.put([]byte("ricki"), []byte("lake"))
@@ -76,9 +76,9 @@ func TestLeafWrite(t *testing.T) {
 	assert.Equal(t, l2.items[2].value, []byte("que"))
 }
 
-// Ensure that a temporary page can split into appropriate subgroups.
+// Ensure that a leaf can split into appropriate subgroups.
 func TestLeafSplit(t *testing.T) {
-	// Create a temp page.
+	// Create a leaf.
 	l := &leaf{items: make(leafItems, 0)}
 	l.put([]byte("00000001"), []byte("0123456701234567"))
 	l.put([]byte("00000002"), []byte("0123456701234567"))
@@ -94,9 +94,9 @@ func TestLeafSplit(t *testing.T) {
 	assert.Equal(t, len(leafs[1].items), 3)
 }
 
-// Ensure that a temporary page with the minimum number of items just returns a single split group.
+// Ensure that a leaf with the minimum number of items just returns a single leaf.
 func TestLeafSplitWithMinKeys(t *testing.T) {
-	// Create a temp page.
+	// Create a leaf.
 	l := &leaf{items: make(leafItems, 0)}
 	l.put([]byte("00000001"), []byte("0123456701234567"))
 	l.put([]byte("00000002"), []byte("0123456701234567"))
@@ -107,9 +107,9 @@ func TestLeafSplitWithMinKeys(t *testing.T) {
 	assert.Equal(t, len(leafs[0].items), 2)
 }
 
-// Ensure that a temporary page that has keys that all fit on a page just returns one split group.
+// Ensure that a leaf that has keys that all fit on a page just returns one leaf.
 func TestLeafSplitFitsInPage(t *testing.T) {
-	// Create a temp page.
+	// Create a leaf.
 	l := &leaf{items: make(leafItems, 0)}
 	l.put([]byte("00000001"), []byte("0123456701234567"))
 	l.put([]byte("00000002"), []byte("0123456701234567"))