diff --git a/consul/state_store.go b/consul/state_store.go
index 7b0808528c..3f1f9a1494 100644
--- a/consul/state_store.go
+++ b/consul/state_store.go
@@ -782,8 +782,51 @@ func (s *StateStore) KVSDeleteTree() error {
 }
 
 // KVSCheckAndSet is used to perform an atomic check-and-set
-func (s *StateStore) KVSCheckAndSet() error {
-	return nil
+func (s *StateStore) KVSCheckAndSet(index uint64, d *structs.DirEntry) (bool, error) {
+	// Start a new txn
+	tx, err := s.kvsTable.StartTxn(false, nil)
+	if err != nil {
+		return false, err
+	}
+	defer tx.Abort()
+
+	// Get the existing node
+	res, err := s.kvsTable.GetTxn(tx, "id", d.Key)
+	if err != nil {
+		return false, err
+	}
+
+	// Get the existing node if any
+	var exist *structs.DirEntry
+	if len(res) > 0 {
+		exist = res[0].(*structs.DirEntry)
+	}
+
+	// Use the ModifyIndex as the constraint. A modify of time of 0
+	// means we are doing a set-if-not-exists, while any other value
+	// means we expect that modify time.
+	if d.ModifyIndex == 0 && exist != nil {
+		return false, nil
+	} else if d.ModifyIndex > 0 && (exist == nil || exist.ModifyIndex != d.ModifyIndex) {
+		return false, nil
+	}
+
+	// Set the create and modify times
+	if exist == nil {
+		d.CreateIndex = index
+	} else {
+		d.CreateIndex = exist.CreateIndex
+	}
+	d.ModifyIndex = index
+
+	if err := s.kvsTable.InsertTxn(tx, d); err != nil {
+		return false, err
+	}
+	if err := s.kvsTable.SetLastIndexTxn(tx, index); err != nil {
+		return false, err
+	}
+	defer s.watch[s.kvsTable].Notify()
+	return true, tx.Commit()
 }
 
 // Snapshot is used to create a point in time snapshot
diff --git a/consul/state_store_test.go b/consul/state_store_test.go
index c2f64dbc7b..c9a26e6763 100644
--- a/consul/state_store_test.go
+++ b/consul/state_store_test.go
@@ -1044,3 +1044,66 @@ func TestKVSDelete(t *testing.T) {
 		t.Fatalf("bad: %v", d)
 	}
 }
+
+func TestKVSCheckAndSet(t *testing.T) {
+	store, err := testStateStore()
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+	defer store.Close()
+
+	// CAS should fail, no entry
+	d := &structs.DirEntry{
+		ModifyIndex: 100,
+		Key:         "/foo",
+		Flags:       42,
+		Value:       []byte("test"),
+	}
+	ok, err := store.KVSCheckAndSet(1000, d)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+	if ok {
+		t.Fatalf("unexpected commit")
+	}
+
+	// Constrain on not-exist, should work
+	d.ModifyIndex = 0
+	ok, err = store.KVSCheckAndSet(1001, d)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+	if !ok {
+		t.Fatalf("expected commit")
+	}
+
+	// Constrain on not-exist, should fail
+	d.ModifyIndex = 0
+	ok, err = store.KVSCheckAndSet(1002, d)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+	if ok {
+		t.Fatalf("unexpected commit")
+	}
+
+	// Constrain on a wrong modify time
+	d.ModifyIndex = 1000
+	ok, err = store.KVSCheckAndSet(1003, d)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+	if ok {
+		t.Fatalf("unexpected commit")
+	}
+
+	// Constrain on a correct modify time
+	d.ModifyIndex = 1001
+	ok, err = store.KVSCheckAndSet(1004, d)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+	if !ok {
+		t.Fatalf("expected commit")
+	}
+}