diff --git a/frame.go b/frame.go
index 8ccf142c..4fb95357 100644
--- a/frame.go
+++ b/frame.go
@@ -85,3 +85,20 @@ func WriteStreamFrame(b *bytes.Buffer, f *StreamFrame) {
 	utils.WriteUint16(b, uint16(len(f.Data)))
 	b.Write(f.Data)
 }
+
+// An AckFrame in QUIC
+type AckFrame struct {
+	LargestObserved uint32 // TODO: change to uint64
+}
+
+// WriteAckFrame writes an ack frame.
+func WriteAckFrame(b *bytes.Buffer, f *AckFrame) {
+	typeByte := uint8(0x48)
+	b.WriteByte(typeByte)
+	b.WriteByte(0x00) // TODO: Entropy accumulation
+	utils.WriteUint32(b, f.LargestObserved)
+	utils.WriteUint16(b, 1) // TODO: Ack delay time
+	b.WriteByte(0x01)       // Just one timestamp
+	b.WriteByte(0x00)       // Largest observed
+	utils.WriteUint32(b, 0) // First timestamp
+}
diff --git a/frame_test.go b/frame_test.go
index 74675e29..26124f88 100644
--- a/frame_test.go
+++ b/frame_test.go
@@ -52,4 +52,16 @@ var _ = Describe("Frame", func() {
 			})
 		})
 	})
+
+	Context("ACK frames", func() {
+		Context("when writing", func() {
+			It("writes simple frames", func() {
+				b := &bytes.Buffer{}
+				WriteAckFrame(b, &AckFrame{
+					LargestObserved: 1,
+				})
+				Expect(b.Bytes()).To(Equal([]byte{0x48, 0, 0x01, 0, 0, 0, 1, 0, 1, 0, 0, 0, 0, 0}))
+			})
+		})
+	})
 })