diff --git a/entropy_accumulator.go b/entropy_accumulator.go
new file mode 100644
index 000000000..cfa2fbdc6
--- /dev/null
+++ b/entropy_accumulator.go
@@ -0,0 +1,16 @@
+package quic
+
+// EntropyAccumulator accumulates the entropy according to the QUIC docs
+type EntropyAccumulator byte
+
+// Add the contribution of the entropy flag of a given packet number
+func (e *EntropyAccumulator) Add(packetNumber uint64, entropyFlag bool) {
+	if entropyFlag {
+		(*e) ^= 0x01 << (packetNumber % 8)
+	}
+}
+
+// Get the byte of entropy
+func (e *EntropyAccumulator) Get() byte {
+	return byte(*e)
+}
diff --git a/entropy_accumulator_test.go b/entropy_accumulator_test.go
new file mode 100644
index 000000000..5b149a227
--- /dev/null
+++ b/entropy_accumulator_test.go
@@ -0,0 +1,25 @@
+package quic
+
+import (
+	. "github.com/onsi/ginkgo"
+	. "github.com/onsi/gomega"
+)
+
+var _ = Describe("EntropyAccumulator", func() {
+	It("initializes as zero", func() {
+		var e EntropyAccumulator
+		Expect(e.Get()).To(BeZero())
+	})
+
+	It("adds entropy", func() {
+		var e EntropyAccumulator
+		e.Add(9, true)
+		Expect(e.Get()).To(Equal(byte(0x02)))
+	})
+
+	It("doesn't add entropy for zero entropy flags", func() {
+		var e EntropyAccumulator
+		e.Add(9, false)
+		Expect(e.Get()).To(BeZero())
+	})
+})
diff --git a/example/main.go b/example/main.go
index 7153a3db8..3cc560f84 100644
--- a/example/main.go
+++ b/example/main.go
@@ -2,7 +2,6 @@ package main
 
 import (
 	"bytes"
-	"errors"
 	"fmt"
 	"net"
 	"os"
@@ -65,9 +64,8 @@ func main() {
 		panic(err)
 	}
 
-	if privateFlag&0x02 > 0 || privateFlag&0x04 > 0 {
-		panic(errors.New("FEC packets are not implemented"))
-	}
+	var entropyAcc quic.EntropyAccumulator
+	entropyAcc.Add(publicHeader.PacketNumber, privateFlag&0x01 > 0)
 
 	frame, err := quic.ParseStreamFrame(r)
 	if err != nil {
@@ -109,6 +107,7 @@ func main() {
 	replyFrame := &bytes.Buffer{}
 	replyFrame.WriteByte(0) // Private header
 	quic.WriteAckFrame(replyFrame, &quic.AckFrame{
+		Entropy:         entropyAcc.Get(),
 		LargestObserved: 1,
 	})
 	quic.WriteStreamFrame(replyFrame, &quic.StreamFrame{
diff --git a/frame.go b/frame.go
index 4fb95357c..0e137c227 100644
--- a/frame.go
+++ b/frame.go
@@ -88,6 +88,7 @@ func WriteStreamFrame(b *bytes.Buffer, f *StreamFrame) {
 
 // An AckFrame in QUIC
 type AckFrame struct {
+	Entropy         byte
 	LargestObserved uint32 // TODO: change to uint64
 }
 
@@ -95,7 +96,7 @@ type AckFrame struct {
 func WriteAckFrame(b *bytes.Buffer, f *AckFrame) {
 	typeByte := uint8(0x48)
 	b.WriteByte(typeByte)
-	b.WriteByte(0x00) // TODO: Entropy accumulation
+	b.WriteByte(f.Entropy)
 	utils.WriteUint32(b, f.LargestObserved)
 	utils.WriteUint16(b, 1) // TODO: Ack delay time
 	b.WriteByte(0x01)       // Just one timestamp
diff --git a/frame_test.go b/frame_test.go
index 26124f88c..7121c7525 100644
--- a/frame_test.go
+++ b/frame_test.go
@@ -58,9 +58,10 @@ var _ = Describe("Frame", func() {
 			It("writes simple frames", func() {
 				b := &bytes.Buffer{}
 				WriteAckFrame(b, &AckFrame{
+					Entropy:         2,
 					LargestObserved: 1,
 				})
-				Expect(b.Bytes()).To(Equal([]byte{0x48, 0, 0x01, 0, 0, 0, 1, 0, 1, 0, 0, 0, 0, 0}))
+				Expect(b.Bytes()).To(Equal([]byte{0x48, 0x02, 0x01, 0, 0, 0, 1, 0, 1, 0, 0, 0, 0, 0}))
 			})
 		})
 	})