diff --git a/syncmap/map.go b/syncmap/map.go
new file mode 100644
index 0000000..1efbcb2
--- /dev/null
+++ b/syncmap/map.go
@@ -0,0 +1,192 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package syncmap provides a concurrent map implementation.
+// It is a prototype for a proposed addition to the sync package
+// in the standard library.
+// (https://golang.org/issue/18177)
+package syncmap
+
+import (
+	"sync"
+	"sync/atomic"
+)
+
+// Map is a concurrent map with amortized-constant-time operations.
+// It is safe for multiple goroutines to call a Map's methods concurrently.
+//
+// The zero Map is valid and empty.
+//
+// A Map must not be copied after first use.
+type Map struct {
+	mu sync.Mutex
+
+	// clean is a copy of the map's contents that will never be overwritten, and
+	// is thus safe for concurrent lookups without additional synchronization.
+	//
+	// A nil clean map indicates that the current map contents are stored in the
+	// dirty field instead.
+	// If clean is non-nil, its contents are up-to-date.
+	//
+	// clean is always safe to load, but must only be stored with mu held.
+	clean atomic.Value // map[interface{}]interface{}
+
+	// dirty is a copy of the map to which all writes occur.
+	//
+	// A nil dirty map indicates that the current map contents are either empty or
+	// stored in the clean field.
+	//
+	// If the dirty map is nil, the next write to the map will initialize it by
+	// making a deep copy of the clean map, then setting the clean map to nil.
+	dirty map[interface{}]interface{}
+
+	// misses counts the number of Load calls for which the clean map was nil
+	// since the last write.
+	//
+	// Once enough Load misses have occurred to cover the cost of a copy, the
+	// dirty map will be promoted to clean and any subsequent writes will make
+	// a new copy.
+	misses int
+}
+
+// Load returns the value stored in the map for a key, or nil if no
+// value is present.
+// The ok result indicates whether value was found in the map.
+func (m *Map) Load(key interface{}) (value interface{}, ok bool) {
+	clean, _ := m.clean.Load().(map[interface{}]interface{})
+	if clean != nil {
+		value, ok = clean[key]
+		return value, ok
+	}
+
+	m.mu.Lock()
+	if m.dirty == nil {
+		clean, _ := m.clean.Load().(map[interface{}]interface{})
+		if clean == nil {
+			// Completely empty — promote to clean immediately.
+			m.clean.Store(map[interface{}]interface{}{})
+		} else {
+			value, ok = clean[key]
+		}
+		m.mu.Unlock()
+		return value, ok
+	}
+	value, ok = m.dirty[key]
+	m.missLocked()
+	m.mu.Unlock()
+	return value, ok
+}
+
+// Store sets the value for a key.
+func (m *Map) Store(key, value interface{}) {
+	m.mu.Lock()
+	m.dirtyLocked()
+	m.dirty[key] = value
+	m.mu.Unlock()
+}
+
+// LoadOrStore returns the existing value for the key if present.
+// Otherwise, it stores and returns the given value.
+// The loaded result is true if the value was loaded, false if stored.
+func (m *Map) LoadOrStore(key, value interface{}) (actual interface{}, loaded bool) {
+	// Avoid locking if it's a clean hit.
+	clean, _ := m.clean.Load().(map[interface{}]interface{})
+	actual, loaded = clean[key]
+	if loaded {
+		return actual, true
+	}
+
+	m.mu.Lock()
+	if m.dirty == nil {
+		// Reload clean in case it changed while we were waiting on m.mu.
+		clean, _ := m.clean.Load().(map[interface{}]interface{})
+		actual, loaded = clean[key]
+		if loaded {
+			m.mu.Unlock()
+			return actual, true
+		}
+	} else {
+		actual, loaded = m.dirty[key]
+		if loaded {
+			m.missLocked()
+			m.mu.Unlock()
+			return actual, true
+		}
+	}
+
+	m.dirtyLocked()
+	m.dirty[key] = value
+	actual = value
+	m.mu.Unlock()
+	return actual, false
+}
+
+// Delete deletes the value for a key.
+func (m *Map) Delete(key interface{}) {
+	m.mu.Lock()
+	m.dirtyLocked()
+	delete(m.dirty, key)
+	m.mu.Unlock()
+}
+
+// Range calls f sequentially for each key and value present in the map.
+// If f returns false, range stops the iteration.
+//
+// Calls to other Map methods may block until Range returns.
+// The function f must not call any other methods on the Map.
+//
+// Range does not necessarily correspond to any consistent snapshot of the Map's
+// contents: no key will be visited more than once, but if the value for any key
+// is stored or deleted concurrently, Range may reflect any mapping for that key
+// from any point during the Range call.
+func (m *Map) Range(f func(key, value interface{}) bool) {
+	clean, _ := m.clean.Load().(map[interface{}]interface{})
+	if clean == nil {
+		m.mu.Lock()
+		if m.dirty == nil {
+			clean, _ = m.clean.Load().(map[interface{}]interface{})
+			if clean == nil {
+				// Completely empty — add an empty map to bypass m.mu next time.
+				m.clean.Store(map[interface{}]interface{}{})
+			}
+		} else {
+			// Range is already O(N), so a call to Range amortizes an entire copy of
+			// the map.  If it is dirty, we can promote it to clean immediately!
+			clean = m.dirty
+			m.clean.Store(clean)
+			m.dirty = nil
+		}
+		m.mu.Unlock()
+	}
+
+	for k, v := range clean {
+		if !f(k, v) {
+			break
+		}
+	}
+}
+
+func (m *Map) missLocked() {
+	if m.misses++; m.misses >= len(m.dirty) {
+		m.clean.Store(m.dirty)
+		m.dirty = nil
+	}
+}
+
+// dirtyLocked prepares the map for a subsequent write.
+// It ensures that the dirty field is non-nil and clean is nil by making a deep
+// copy of clean.
+func (m *Map) dirtyLocked() {
+	m.misses = 0
+	if m.dirty != nil {
+		return
+	}
+
+	clean, _ := m.clean.Load().(map[interface{}]interface{})
+	m.dirty = make(map[interface{}]interface{}, len(clean))
+	for k, v := range clean {
+		m.dirty[k] = v
+	}
+	m.clean.Store(map[interface{}]interface{}(nil))
+}
diff --git a/syncmap/map_bench_test.go b/syncmap/map_bench_test.go
new file mode 100644
index 0000000..ac06274
--- /dev/null
+++ b/syncmap/map_bench_test.go
@@ -0,0 +1,199 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package syncmap_test
+
+import (
+	"fmt"
+	"reflect"
+	"sync/atomic"
+	"testing"
+
+	"golang.org/x/sync/syncmap"
+)
+
+type bench struct {
+	setup func(*testing.B, mapInterface)
+	perG  func(b *testing.B, pb *testing.PB, i int, m mapInterface)
+}
+
+func benchMap(b *testing.B, bench bench) {
+	for _, m := range [...]mapInterface{&DeepCopyMap{}, &RWMutexMap{}, &syncmap.Map{}} {
+		b.Run(fmt.Sprintf("%T", m), func(b *testing.B) {
+			m = reflect.New(reflect.TypeOf(m).Elem()).Interface().(mapInterface)
+			if bench.setup != nil {
+				bench.setup(b, m)
+			}
+
+			b.ResetTimer()
+
+			var i int64
+			b.RunParallel(func(pb *testing.PB) {
+				id := int(atomic.AddInt64(&i, 1) - 1)
+				bench.perG(b, pb, id*b.N, m)
+			})
+		})
+	}
+}
+
+func BenchmarkLoadMostlyHits(b *testing.B) {
+	const hits, misses = 1023, 1
+
+	benchMap(b, bench{
+		setup: func(_ *testing.B, m mapInterface) {
+			for i := 0; i < hits; i++ {
+				m.LoadOrStore(i, i)
+			}
+			// Prime the map to get it into a steady state.
+			for i := 0; i < hits*2; i++ {
+				m.Load(i % hits)
+			}
+		},
+
+		perG: func(b *testing.B, pb *testing.PB, i int, m mapInterface) {
+			for ; pb.Next(); i++ {
+				m.Load(i % (hits + misses))
+			}
+		},
+	})
+}
+
+func BenchmarkLoadMostlyMisses(b *testing.B) {
+	const hits, misses = 1, 1023
+
+	benchMap(b, bench{
+		setup: func(_ *testing.B, m mapInterface) {
+			for i := 0; i < hits; i++ {
+				m.LoadOrStore(i, i)
+			}
+			// Prime the map to get it into a steady state.
+			for i := 0; i < hits*2; i++ {
+				m.Load(i % hits)
+			}
+		},
+
+		perG: func(b *testing.B, pb *testing.PB, i int, m mapInterface) {
+			for ; pb.Next(); i++ {
+				m.Load(i % (hits + misses))
+			}
+		},
+	})
+}
+
+func BenchmarkLoadOrStoreBalanced(b *testing.B) {
+	const hits, misses = 128, 128
+
+	benchMap(b, bench{
+		setup: func(b *testing.B, m mapInterface) {
+			if _, ok := m.(*DeepCopyMap); ok {
+				b.Skip("DeepCopyMap has quadratic running time.")
+			}
+			for i := 0; i < hits; i++ {
+				m.LoadOrStore(i, i)
+			}
+			// Prime the map to get it into a steady state.
+			for i := 0; i < hits*2; i++ {
+				m.Load(i % hits)
+			}
+		},
+
+		perG: func(b *testing.B, pb *testing.PB, i int, m mapInterface) {
+			for ; pb.Next(); i++ {
+				j := i % (hits + misses)
+				if j < hits {
+					if _, ok := m.LoadOrStore(j, i); !ok {
+						b.Fatalf("unexpected miss for %v", j)
+					}
+				} else {
+					if v, loaded := m.LoadOrStore(i, i); loaded {
+						b.Fatalf("failed to store %v: existing value %v", i, v)
+					}
+				}
+			}
+		},
+	})
+}
+
+func BenchmarkLoadOrStoreUnique(b *testing.B) {
+	benchMap(b, bench{
+		setup: func(b *testing.B, m mapInterface) {
+			if _, ok := m.(*DeepCopyMap); ok {
+				b.Skip("DeepCopyMap has quadratic running time.")
+			}
+		},
+
+		perG: func(b *testing.B, pb *testing.PB, i int, m mapInterface) {
+			for ; pb.Next(); i++ {
+				m.LoadOrStore(i, i)
+			}
+		},
+	})
+}
+
+func BenchmarkLoadOrStoreCollision(b *testing.B) {
+	benchMap(b, bench{
+		setup: func(_ *testing.B, m mapInterface) {
+			m.LoadOrStore(0, 0)
+		},
+
+		perG: func(b *testing.B, pb *testing.PB, i int, m mapInterface) {
+			for ; pb.Next(); i++ {
+				m.LoadOrStore(0, 0)
+			}
+		},
+	})
+}
+
+// BenchmarkAdversarialAlloc tests performance when we store a new value
+// immediately whenever the map is promoted to clean.
+//
+// This forces the Load calls to always acquire the map's mutex.
+func BenchmarkAdversarialAlloc(b *testing.B) {
+	benchMap(b, bench{
+		perG: func(b *testing.B, pb *testing.PB, i int, m mapInterface) {
+			var stores, loadsSinceStore int64
+			for ; pb.Next(); i++ {
+				m.Load(i)
+				if loadsSinceStore++; loadsSinceStore > stores {
+					m.LoadOrStore(i, stores)
+					loadsSinceStore = 0
+					stores++
+				}
+			}
+		},
+	})
+}
+
+// BenchmarkAdversarialDelete tests performance when we delete and restore a
+// value immediately after a large map has been promoted.
+//
+// This forces the Load calls to always acquire the map's mutex and periodically
+// makes a full copy of the map despite changing only one entry.
+func BenchmarkAdversarialDelete(b *testing.B) {
+	const mapSize = 1 << 10
+
+	benchMap(b, bench{
+		setup: func(_ *testing.B, m mapInterface) {
+			for i := 0; i < mapSize; i++ {
+				m.Store(i, i)
+			}
+		},
+
+		perG: func(b *testing.B, pb *testing.PB, i int, m mapInterface) {
+			for ; pb.Next(); i++ {
+				m.Load(i)
+
+				if i%mapSize == 0 {
+					var key int
+					m.Range(func(k, _ interface{}) bool {
+						key = k.(int)
+						return false
+					})
+					m.Delete(key)
+					m.Store(key, key)
+				}
+			}
+		},
+	})
+}
diff --git a/syncmap/map_reference_test.go b/syncmap/map_reference_test.go
new file mode 100644
index 0000000..f3a4977
--- /dev/null
+++ b/syncmap/map_reference_test.go
@@ -0,0 +1,142 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package syncmap_test
+
+import (
+	"sync"
+	"sync/atomic"
+)
+
+// This file contains reference map implementations for unit-tests.
+
+// mapInterface is the interface Map implements.
+type mapInterface interface {
+	Load(interface{}) (interface{}, bool)
+	Store(key, value interface{})
+	LoadOrStore(key, value interface{}) (actual interface{}, loaded bool)
+	Delete(interface{})
+	Range(func(key, value interface{}) (shouldContinue bool))
+}
+
+// RWMutexMap is an implementation of mapInterface using a sync.RWMutex.
+type RWMutexMap struct {
+	mu    sync.RWMutex
+	dirty map[interface{}]interface{}
+}
+
+func (m *RWMutexMap) Load(key interface{}) (value interface{}, ok bool) {
+	m.mu.RLock()
+	value, ok = m.dirty[key]
+	m.mu.RUnlock()
+	return
+}
+
+func (m *RWMutexMap) Store(key, value interface{}) {
+	m.mu.Lock()
+	if m.dirty == nil {
+		m.dirty = make(map[interface{}]interface{})
+	}
+	m.dirty[key] = value
+	m.mu.Unlock()
+}
+
+func (m *RWMutexMap) LoadOrStore(key, value interface{}) (actual interface{}, loaded bool) {
+	m.mu.Lock()
+	actual, loaded = m.dirty[key]
+	if !loaded {
+		actual = value
+		if m.dirty == nil {
+			m.dirty = make(map[interface{}]interface{})
+		}
+		m.dirty[key] = value
+	}
+	m.mu.Unlock()
+	return actual, loaded
+}
+
+func (m *RWMutexMap) Delete(key interface{}) {
+	m.mu.Lock()
+	delete(m.dirty, key)
+	m.mu.Unlock()
+}
+
+func (m *RWMutexMap) Range(f func(key, value interface{}) (shouldContinue bool)) {
+	m.mu.RLock()
+	defer m.mu.RUnlock()
+	for k, v := range m.dirty {
+		if !f(k, v) {
+			break
+		}
+	}
+}
+
+// DeepCopyMap is an implementation of mapInterface using a Mutex and
+// atomic.Value.  It makes deep copies of the map on every write to avoid
+// acquiring the Mutex in Load.
+type DeepCopyMap struct {
+	mu    sync.Mutex
+	clean atomic.Value
+}
+
+func (m *DeepCopyMap) Load(key interface{}) (value interface{}, ok bool) {
+	clean, _ := m.clean.Load().(map[interface{}]interface{})
+	value, ok = clean[key]
+	return value, ok
+}
+
+func (m *DeepCopyMap) Store(key, value interface{}) {
+	m.mu.Lock()
+	dirty := m.dirty()
+	dirty[key] = value
+	m.clean.Store(dirty)
+	m.mu.Unlock()
+}
+
+func (m *DeepCopyMap) LoadOrStore(key, value interface{}) (actual interface{}, loaded bool) {
+	clean, _ := m.clean.Load().(map[interface{}]interface{})
+	actual, loaded = clean[key]
+	if loaded {
+		return actual, loaded
+	}
+
+	m.mu.Lock()
+	// Reload clean in case it changed while we were waiting on m.mu.
+	clean, _ = m.clean.Load().(map[interface{}]interface{})
+	actual, loaded = clean[key]
+	if !loaded {
+		dirty := m.dirty()
+		dirty[key] = value
+		actual = value
+		m.clean.Store(dirty)
+	}
+	m.mu.Unlock()
+	return actual, loaded
+}
+
+func (m *DeepCopyMap) Delete(key interface{}) {
+	m.mu.Lock()
+	dirty := m.dirty()
+	delete(dirty, key)
+	m.clean.Store(dirty)
+	m.mu.Unlock()
+}
+
+func (m *DeepCopyMap) Range(f func(key, value interface{}) (shouldContinue bool)) {
+	clean, _ := m.clean.Load().(map[interface{}]interface{})
+	for k, v := range clean {
+		if !f(k, v) {
+			break
+		}
+	}
+}
+
+func (m *DeepCopyMap) dirty() map[interface{}]interface{} {
+	clean, _ := m.clean.Load().(map[interface{}]interface{})
+	dirty := make(map[interface{}]interface{}, len(clean)+1)
+	for k, v := range clean {
+		dirty[k] = v
+	}
+	return dirty
+}
diff --git a/syncmap/map_test.go b/syncmap/map_test.go
new file mode 100644
index 0000000..54448a0
--- /dev/null
+++ b/syncmap/map_test.go
@@ -0,0 +1,116 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package syncmap_test
+
+import (
+	"fmt"
+	"math/rand"
+	"reflect"
+	"testing"
+	"testing/quick"
+
+	"golang.org/x/sync/syncmap"
+)
+
+// mapCall is a quick.Generator for calls on mapInterface.
+type mapCall struct {
+	key   interface{}
+	apply func(mapInterface) (interface{}, bool)
+	desc  string
+}
+
+type mapResult struct {
+	value interface{}
+	ok    bool
+}
+
+var stringType = reflect.TypeOf("")
+
+func randValue(r *rand.Rand) interface{} {
+	k, ok := quick.Value(stringType, r)
+	if !ok {
+		panic(fmt.Sprintf("quick.Value(%v, _) failed", stringType))
+	}
+	return k.Interface()
+}
+
+func (mapCall) Generate(r *rand.Rand, size int) reflect.Value {
+	k := randValue(r)
+
+	var (
+		app  func(mapInterface) (interface{}, bool)
+		desc string
+	)
+	switch rand.Intn(4) {
+	case 0:
+		app = func(m mapInterface) (interface{}, bool) {
+			return m.Load(k)
+		}
+		desc = fmt.Sprintf("Load(%q)", k)
+
+	case 1:
+		v := randValue(r)
+		app = func(m mapInterface) (interface{}, bool) {
+			m.Store(k, v)
+			return nil, false
+		}
+		desc = fmt.Sprintf("Store(%q, %q)", k, v)
+
+	case 2:
+		v := randValue(r)
+		app = func(m mapInterface) (interface{}, bool) {
+			return m.LoadOrStore(k, v)
+		}
+		desc = fmt.Sprintf("LoadOrStore(%q, %q)", k, v)
+
+	case 3:
+		app = func(m mapInterface) (interface{}, bool) {
+			m.Delete(k)
+			return nil, false
+		}
+		desc = fmt.Sprintf("Delete(%q)", k)
+	}
+
+	return reflect.ValueOf(mapCall{k, app, desc})
+}
+
+func applyCalls(m mapInterface, calls []mapCall) (results []mapResult, final map[interface{}]interface{}) {
+	for _, c := range calls {
+		v, ok := c.apply(m)
+		results = append(results, mapResult{v, ok})
+	}
+
+	final = make(map[interface{}]interface{})
+	m.Range(func(k, v interface{}) bool {
+		final[k] = v
+		return true
+	})
+
+	return results, final
+}
+
+func applyMap(calls []mapCall) ([]mapResult, map[interface{}]interface{}) {
+	return applyCalls(new(syncmap.Map), calls)
+}
+
+func applyRWMutexMap(calls []mapCall) ([]mapResult, map[interface{}]interface{}) {
+	return applyCalls(new(RWMutexMap), calls)
+}
+
+func applyDeepCopyMap(calls []mapCall) ([]mapResult, map[interface{}]interface{}) {
+	return applyCalls(new(DeepCopyMap), calls)
+}
+
+func TestMapMatchesRWMutex(t *testing.T) {
+	if err := quick.CheckEqual(applyMap, applyRWMutexMap, nil); err != nil {
+		t.Error(err)
+	}
+}
+
+func TestMapMatchesDeepCopy(t *testing.T) {
+	if err := quick.CheckEqual(applyMap, applyRWMutexMap, nil); err != nil {
+		t.Error(err)
+	}
+}