diff --git a/db.go b/db.go
index 65879ec3..8592d836 100644
--- a/db.go
+++ b/db.go
@@ -15,6 +15,9 @@ const (
 
 const minPageSize = 0x1000
 
+const minMmapSize = 1 << 22 // 4MB
+const maxMmapStep = 1 << 30 // 1GB
+
 type DB struct {
 	os            _os
 	syscall       _syscall
@@ -98,7 +101,7 @@ func (db *DB) Open(path string, mode os.FileMode) error {
 	}
 
 	// Memory map the data file.
-	if err := db.mmap(); err != nil {
+	if err := db.mmap(0); err != nil {
 		db.close()
 		return err
 	}
@@ -113,7 +116,19 @@ func (db *DB) Open(path string, mode os.FileMode) error {
 }
 
 // mmap opens the underlying memory-mapped file and initializes the meta references.
-func (db *DB) mmap() error {
+// minsz is the minimum size that the new mmap can be.
+func (db *DB) mmap(minsz int) error {
+	db.mmaplock.Lock()
+	defer db.mmaplock.Unlock()
+
+	// Dereference all mmap references before unmapping.
+	if db.rwtransaction != nil {
+		db.rwtransaction.dereference()
+	}
+
+	// Unmap existing data before continuing.
+	db.munmap()
+
 	info, err := db.file.Stat()
 	if err != nil {
 		return &Error{"mmap stat error", err}
@@ -121,8 +136,12 @@ func (db *DB) mmap() error {
 		return &Error{"file size too small", err}
 	}
 
-	// TODO(benbjohnson): Determine appropriate mmap size by db size.
-	size := 2 << 30
+	// Ensure the size is at least the minimum size.
+	var size = int(info.Size())
+	if size < minsz {
+		size = minsz
+	}
+	size = db.mmapSize(minsz)
 
 	// Memory-map the data file as a byte slice.
 	if db.data, err = db.syscall.Mmap(int(db.file.Fd()), 0, size, syscall.PROT_READ, syscall.MAP_SHARED); err != nil {
@@ -144,6 +163,35 @@ func (db *DB) mmap() error {
 	return nil
 }
 
+// munmap unmaps the data file from memory.
+func (db *DB) munmap() {
+	if db.data != nil {
+		if err := db.syscall.Munmap(db.data); err != nil {
+			panic("unmap error: " + err.Error())
+		}
+		db.data = nil
+	}
+}
+
+// mmapSize determines the appropriate size for the mmap given the current size
+// of the database. The minimum size is 4MB and doubles until it reaches 1GB.
+func (db *DB) mmapSize(size int) int {
+	if size < minMmapSize {
+		return minMmapSize
+	} else if size < maxMmapStep {
+		size *= 2
+	} else {
+		size += maxMmapStep
+	}
+
+	// Ensure that the mmap size is a multiple of the page size.
+	if (size % db.pageSize) != 0 {
+		size = ((size / db.pageSize) + 1) * db.pageSize
+	}
+
+	return size
+}
+
 // init creates a new database file and initializes its meta pages.
 func (db *DB) init() error {
 	// Set the page size to the OS page size.
@@ -196,8 +244,14 @@ func (db *DB) Close() {
 }
 
 func (db *DB) close() {
-	// TODO: Undo everything in Open().
+	// Wait for pending transactions before closing and unmapping the data.
+	// db.mmaplock.Lock()
+	// defer db.mmaplock.Unlock()
+
+	// TODO(benbjohnson): Undo everything in Open().
 	db.freelist = nil
+
+	db.munmap()
 }
 
 // Transaction creates a read-only transaction.
@@ -206,6 +260,11 @@ func (db *DB) Transaction() (*Transaction, error) {
 	db.metalock.Lock()
 	defer db.metalock.Unlock()
 
+	// Obtain a read-only lock on the mmap. When the mmap is remapped it will
+	// obtain a write lock so all transactions must finish before it can be
+	// remapped.
+	db.mmaplock.RLock()
+
 	// Exit if the database is not open yet.
 	if !db.opened {
 		return nil, DatabaseNotOpenError
@@ -260,6 +319,9 @@ func (db *DB) removeTransaction(t *Transaction) {
 	db.metalock.Lock()
 	defer db.metalock.Unlock()
 
+	// Release the read lock on the mmap.
+	db.mmaplock.RUnlock()
+
 	// Remove the transaction.
 	for i, txn := range db.transactions {
 		if txn == t {
@@ -412,7 +474,7 @@ func (db *DB) meta() *meta {
 }
 
 // allocate returns a contiguous block of memory starting at a given page.
-func (db *DB) allocate(count int) *page {
+func (db *DB) allocate(count int) (*page, error) {
 	// Allocate a temporary buffer for the page.
 	buf := make([]byte, count*db.pageSize)
 	p := (*page)(unsafe.Pointer(&buf[0]))
@@ -420,16 +482,22 @@ func (db *DB) allocate(count int) *page {
 
 	// Use pages from the freelist if they are available.
 	if p.id = db.freelist.allocate(count); p.id != 0 {
-		return p
+		return p, nil
 	}
 
-	// TODO(benbjohnson): Resize mmap().
-
-	// If there are no free pages then allocate from the end of the file.
+	// Resize mmap() if we're at the end.
 	p.id = db.rwtransaction.meta.pgid
+	var minsz = int((p.id+pgid(count))+1) * db.pageSize
+	if minsz >= len(db.data) {
+		if err := db.mmap(minsz); err != nil {
+			return nil, &Error{"mmap allocate error", err}
+		}
+	}
+
+	// Move the page id high water mark.
 	db.rwtransaction.meta.pgid += pgid(count)
 
-	return p
+	return p, nil
 }
 
 // sync flushes the file descriptor to disk.
diff --git a/db_test.go b/db_test.go
index 8211eac7..db578256 100644
--- a/db_test.go
+++ b/db_test.go
@@ -187,6 +187,18 @@ func TestDBWriteFail(t *testing.T) {
 	t.Skip("pending") // TODO(benbjohnson)
 }
 
+// Ensure that the mmap grows appropriately.
+func TestDBMmapSize(t *testing.T) {
+	db := &DB{pageSize: 4096}
+	assert.Equal(t, db.mmapSize(0), minMmapSize)
+	assert.Equal(t, db.mmapSize(16384), minMmapSize)
+	assert.Equal(t, db.mmapSize(minMmapSize-1), minMmapSize)
+	assert.Equal(t, db.mmapSize(minMmapSize), minMmapSize*2)
+	assert.Equal(t, db.mmapSize(10000000), 20000768)
+	assert.Equal(t, db.mmapSize((1<<30)-1), 2147483648)
+	assert.Equal(t, db.mmapSize(1<<30), 1<<31)
+}
+
 // withDB executes a function with a database reference.
 func withDB(fn func(*DB, string)) {
 	f, _ := ioutil.TempFile("", "bolt-")
diff --git a/node.go b/node.go
index 11b5e2f0..1460f529 100644
--- a/node.go
+++ b/node.go
@@ -161,10 +161,8 @@ func (n *node) write(p *page) {
 	// Initialize page.
 	if n.isLeaf {
 		p.flags |= p_leaf
-		// warn("∑", p.id, "leaf")
 	} else {
 		p.flags |= p_branch
-		// warn("∑", p.id, "branch")
 	}
 	p.count = uint16(len(n.inodes))
 
@@ -177,13 +175,11 @@ func (n *node) write(p *page) {
 			elem.pos = uint32(uintptr(unsafe.Pointer(&b[0])) - uintptr(unsafe.Pointer(elem)))
 			elem.ksize = uint32(len(item.key))
 			elem.vsize = uint32(len(item.value))
-			// warn("  »", string(item.key), "->", string(item.value))
 		} else {
 			elem := p.branchPageElement(uint16(i))
 			elem.pos = uint32(uintptr(unsafe.Pointer(&b[0])) - uintptr(unsafe.Pointer(elem)))
 			elem.ksize = uint32(len(item.key))
 			elem.pgid = item.pgid
-			// warn("  »", string(item.key))
 		}
 
 		// Write data for the element to the end of the page.
@@ -341,6 +337,26 @@ func (n *node) rebalance() {
 	n.parent.rebalance()
 }
 
+// dereference causes the node to copy all its inode key/value references to heap memory.
+// This is required when the mmap is reallocated so inodes are not pointing to stale data.
+func (n *node) dereference() {
+	key := make([]byte, len(n.key))
+	copy(key, n.key)
+	n.key = key
+
+	for i, _ := range n.inodes {
+		inode := &n.inodes[i]
+
+		key := make([]byte, len(inode.key))
+		copy(key, inode.key)
+		inode.key = key
+
+		value := make([]byte, len(inode.value))
+		copy(value, inode.value)
+		inode.value = value
+	}
+}
+
 // nodesByDepth sorts a list of branches by deepest first.
 type nodesByDepth []*node
 
diff --git a/rwtransaction.go b/rwtransaction.go
index 5058dd3b..b96df51c 100644
--- a/rwtransaction.go
+++ b/rwtransaction.go
@@ -9,7 +9,8 @@ import (
 // Only one read/write transaction can be active for a DB at a time.
 type RWTransaction struct {
 	Transaction
-	nodes map[pgid]*node
+	nodes   map[pgid]*node
+	pending []*node
 }
 
 // init initializes the transaction.
@@ -35,7 +36,10 @@ func (t *RWTransaction) CreateBucket(name string) error {
 	}
 
 	// Create a blank root leaf page.
-	p := t.allocate(1)
+	p, err := t.allocate(1)
+	if err != nil {
+		return err
+	}
 	p.flags = p_leaf
 
 	// Add bucket to buckets page.
@@ -100,14 +104,15 @@ func (t *RWTransaction) Commit() error {
 
 	// TODO(benbjohnson): Use vectorized I/O to write out dirty pages.
 
-	// TODO(benbjohnson): Move rebalancing to occur immediately after deletion (?).
-
 	// Rebalance and spill data onto dirty pages.
 	t.rebalance()
 	t.spill()
 
 	// Spill buckets page.
-	p := t.allocate((t.buckets.size() / t.db.pageSize) + 1)
+	p, err := t.allocate((t.buckets.size() / t.db.pageSize) + 1)
+	if err != nil {
+		return err
+	}
 	t.buckets.write(p)
 
 	// Write dirty pages to disk.
@@ -135,13 +140,16 @@ func (t *RWTransaction) close() {
 }
 
 // allocate returns a contiguous block of memory starting at a given page.
-func (t *RWTransaction) allocate(count int) *page {
-	p := t.db.allocate(count)
+func (t *RWTransaction) allocate(count int) (*page, error) {
+	p, err := t.db.allocate(count)
+	if err != nil {
+		return nil, err
+	}
 
 	// Save to our page cache.
 	t.pages[p.id] = p
 
-	return p
+	return p, nil
 }
 
 // rebalance attempts to balance all nodes.
@@ -152,7 +160,7 @@ func (t *RWTransaction) rebalance() {
 }
 
 // spill writes all the nodes to dirty pages.
-func (t *RWTransaction) spill() {
+func (t *RWTransaction) spill() error {
 	// Keep track of the current root nodes.
 	// We will update this at the end once all nodes are created.
 	type root struct {
@@ -180,6 +188,7 @@ func (t *RWTransaction) spill() {
 		// Split nodes into appropriate sized nodes.
 		// The first node in this list will be a reference to n to preserve ancestry.
 		newNodes := n.split(t.db.pageSize)
+		t.pending = newNodes
 
 		// If this is a root node that split then create a parent node.
 		if n.parent == nil && len(newNodes) > 1 {
@@ -195,7 +204,10 @@ func (t *RWTransaction) spill() {
 		// Write nodes to dirty pages.
 		for i, newNode := range newNodes {
 			// Allocate contiguous space for the node.
-			p := t.allocate((newNode.size() / t.db.pageSize) + 1)
+			p, err := t.allocate((newNode.size() / t.db.pageSize) + 1)
+			if err != nil {
+				return err
+			}
 
 			// Write the node to the page.
 			newNode.write(p)
@@ -215,6 +227,8 @@ func (t *RWTransaction) spill() {
 				newNode.parent.put(oldKey, newNode.inodes[0].key, nil, newNode.pgid)
 			}
 		}
+
+		t.pending = nil
 	}
 
 	// Update roots with new roots.
@@ -224,12 +238,12 @@ func (t *RWTransaction) spill() {
 
 	// Clear out nodes now that they are all spilled.
 	t.nodes = make(map[pgid]*node)
+
+	return nil
 }
 
 // write writes any dirty pages to disk.
 func (t *RWTransaction) write() error {
-	// TODO(benbjohnson): If our last page id is greater than the mmap size then lock the DB and resize.
-
 	// Sort pages by id.
 	pages := make(pages, 0, len(t.pages))
 	for _, p := range t.pages {
@@ -247,6 +261,9 @@ func (t *RWTransaction) write() error {
 		}
 	}
 
+	// Clear out page cache.
+	t.pages = make(map[pgid]*page)
+
 	return nil
 }
 
@@ -280,3 +297,14 @@ func (t *RWTransaction) node(pgid pgid, parent *node) *node {
 
 	return n
 }
+
+// dereference removes all references to the old mmap.
+func (t *RWTransaction) dereference() {
+	for _, n := range t.nodes {
+		n.dereference()
+	}
+
+	for _, n := range t.pending {
+		n.dereference()
+	}
+}
diff --git a/syscall_darwin.go b/syscall_darwin.go
index de341937..cb9a20c9 100644
--- a/syscall_darwin.go
+++ b/syscall_darwin.go
@@ -6,11 +6,15 @@ import (
 
 type _syscall interface {
 	Mmap(fd int, offset int64, length int, prot int, flags int) (data []byte, err error)
+	Munmap([]byte) error
 }
 
 type syssyscall struct{}
 
 func (o *syssyscall) Mmap(fd int, offset int64, length int, prot int, flags int) (data []byte, err error) {
-	// err = (EACCES, EBADF, EINVAL, ENODEV, ENOMEM, ENXIO, EOVERFLOW)
 	return syscall.Mmap(fd, offset, length, prot, flags)
 }
+
+func (o *syssyscall) Munmap(b []byte) error {
+	return syscall.Munmap(b)
+}
diff --git a/syscall_darwin_test.go b/syscall_darwin_test.go
index 9e64cf73..6b468f63 100644
--- a/syscall_darwin_test.go
+++ b/syscall_darwin_test.go
@@ -12,3 +12,8 @@ func (m *mocksyscall) Mmap(fd int, offset int64, length int, prot int, flags int
 	args := m.Called(fd, offset, length, prot, flags)
 	return args.Get(0).([]byte), args.Error(1)
 }
+
+func (m *mocksyscall) Munmap(b []byte) error {
+	args := m.Called(b)
+	return args.Error(0)
+}
diff --git a/syscall_linux.go b/syscall_linux.go
index de341937..65daad43 100644
--- a/syscall_linux.go
+++ b/syscall_linux.go
@@ -6,6 +6,7 @@ import (
 
 type _syscall interface {
 	Mmap(fd int, offset int64, length int, prot int, flags int) (data []byte, err error)
+	Munmap([]byte) error
 }
 
 type syssyscall struct{}
@@ -14,3 +15,7 @@ func (o *syssyscall) Mmap(fd int, offset int64, length int, prot int, flags int)
 	// err = (EACCES, EBADF, EINVAL, ENODEV, ENOMEM, ENXIO, EOVERFLOW)
 	return syscall.Mmap(fd, offset, length, prot, flags)
 }
+
+func (o *syssyscall) Munmap(b []byte) error {
+	return syscall.Munmap(b)
+}
diff --git a/syscall_linux_test.go b/syscall_linux_test.go
index 9e64cf73..6b468f63 100644
--- a/syscall_linux_test.go
+++ b/syscall_linux_test.go
@@ -12,3 +12,8 @@ func (m *mocksyscall) Mmap(fd int, offset int64, length int, prot int, flags int
 	args := m.Called(fd, offset, length, prot, flags)
 	return args.Get(0).([]byte), args.Error(1)
 }
+
+func (m *mocksyscall) Munmap(b []byte) error {
+	args := m.Called(b)
+	return args.Error(0)
+}