diff --git a/floodsub.go b/floodsub.go
index fc67d22f..b1247d5a 100644
--- a/floodsub.go
+++ b/floodsub.go
@@ -17,7 +17,12 @@ import (
 	timecache "github.com/whyrusleeping/timecache"
 )
 
-const ID = protocol.ID("/floodsub/1.0.0")
+const (
+	ID                         = protocol.ID("/floodsub/1.0.0")
+	defaultValidateTimeout     = 150 * time.Millisecond
+	defaultValidateConcurrency = 100
+	defaultValidateThrottle    = 8192
+)
 
 var log = logging.Logger("floodsub")
 
@@ -54,6 +59,18 @@ type PubSub struct {
 	// topics tracks which topics each of our peers are subscribed to
 	topics map[string]map[peer.ID]struct{}
 
+	// sendMsg handles messages that have been validated
+	sendMsg chan *sendReq
+
+	// addVal handles validator registration requests
+	addVal chan *addValReq
+
+	// topicVals tracks per topic validators
+	topicVals map[string]*topicVal
+
+	// validateThrottle limits the number of active validation goroutines
+	validateThrottle chan struct{}
+
 	peers        map[peer.ID]chan *RPC
 	seenMessages *timecache.TimeCache
 
@@ -78,24 +95,37 @@ type RPC struct {
 	from peer.ID
 }
 
+type Option func(*PubSub) error
+
 // NewFloodSub returns a new FloodSub management object
-func NewFloodSub(ctx context.Context, h host.Host) *PubSub {
+func NewFloodSub(ctx context.Context, h host.Host, opts ...Option) (*PubSub, error) {
 	ps := &PubSub{
-		host:         h,
-		ctx:          ctx,
-		incoming:     make(chan *RPC, 32),
-		publish:      make(chan *Message),
-		newPeers:     make(chan inet.Stream),
-		peerDead:     make(chan peer.ID),
-		cancelCh:     make(chan *Subscription),
-		getPeers:     make(chan *listPeerReq),
-		addSub:       make(chan *addSubReq),
-		getTopics:    make(chan *topicReq),
-		myTopics:     make(map[string]map[*Subscription]struct{}),
-		topics:       make(map[string]map[peer.ID]struct{}),
-		peers:        make(map[peer.ID]chan *RPC),
-		seenMessages: timecache.NewTimeCache(time.Second * 30),
-		counter:      uint64(time.Now().UnixNano()),
+		host:             h,
+		ctx:              ctx,
+		incoming:         make(chan *RPC, 32),
+		publish:          make(chan *Message),
+		newPeers:         make(chan inet.Stream),
+		peerDead:         make(chan peer.ID),
+		cancelCh:         make(chan *Subscription),
+		getPeers:         make(chan *listPeerReq),
+		addSub:           make(chan *addSubReq),
+		getTopics:        make(chan *topicReq),
+		sendMsg:          make(chan *sendReq, 32),
+		addVal:           make(chan *addValReq),
+		validateThrottle: make(chan struct{}, defaultValidateThrottle),
+		myTopics:         make(map[string]map[*Subscription]struct{}),
+		topics:           make(map[string]map[peer.ID]struct{}),
+		peers:            make(map[peer.ID]chan *RPC),
+		topicVals:        make(map[string]*topicVal),
+		seenMessages:     timecache.NewTimeCache(time.Second * 30),
+		counter:          uint64(time.Now().UnixNano()),
+	}
+
+	for _, opt := range opts {
+		err := opt(ps)
+		if err != nil {
+			return nil, err
+		}
 	}
 
 	h.SetStreamHandler(ID, ps.handleNewStream)
@@ -103,7 +133,14 @@ func NewFloodSub(ctx context.Context, h host.Host) *PubSub {
 
 	go ps.processLoop(ctx)
 
-	return ps
+	return ps, nil
+}
+
+func WithValidateThrottle(n int) Option {
+	return func(ps *PubSub) error {
+		ps.validateThrottle = make(chan struct{}, n)
+		return nil
+	}
 }
 
 // processLoop handles all inputs arriving on the channels
@@ -176,7 +213,15 @@ func (p *PubSub) processLoop(ctx context.Context) {
 				continue
 			}
 		case msg := <-p.publish:
-			p.maybePublishMessage(p.host.ID(), msg.Message)
+			vals := p.getValidators(msg)
+			p.pushMsg(vals, p.host.ID(), msg)
+
+		case req := <-p.sendMsg:
+			p.maybePublishMessage(req.from, req.msg.Message)
+
+		case req := <-p.addVal:
+			p.addValidator(req)
+
 		case <-ctx.Done():
 			log.Info("pubsub processloop shutting down")
 			return
@@ -210,24 +255,22 @@ func (p *PubSub) handleRemoveSubscription(sub *Subscription) {
 // subscribes to the topic.
 // Only called from processLoop.
 func (p *PubSub) handleAddSubscription(req *addSubReq) {
-	subs := p.myTopics[req.topic]
+	sub := req.sub
+	subs := p.myTopics[sub.topic]
 
 	// announce we want this topic
 	if len(subs) == 0 {
-		p.announce(req.topic, true)
+		p.announce(sub.topic, true)
 	}
 
 	// make new if not there
 	if subs == nil {
-		p.myTopics[req.topic] = make(map[*Subscription]struct{})
-		subs = p.myTopics[req.topic]
+		p.myTopics[sub.topic] = make(map[*Subscription]struct{})
+		subs = p.myTopics[sub.topic]
 	}
 
-	sub := &Subscription{
-		ch:       make(chan *Message, 32),
-		topic:    req.topic,
-		cancelCh: p.cancelCh,
-	}
+	sub.ch = make(chan *Message, 32)
+	sub.cancelCh = p.cancelCh
 
 	p.myTopics[sub.topic][sub] = struct{}{}
 
@@ -314,8 +357,11 @@ func (p *PubSub) handleIncomingRPC(rpc *RPC) error {
 			continue
 		}
 
-		p.maybePublishMessage(rpc.from, pmsg)
+		msg := &Message{pmsg}
+		vals := p.getValidators(msg)
+		p.pushMsg(vals, rpc.from, msg)
 	}
+
 	return nil
 }
 
@@ -324,6 +370,75 @@ func msgID(pmsg *pb.Message) string {
 	return string(pmsg.GetFrom()) + string(pmsg.GetSeqno())
 }
 
+// pushMsg pushes a message performing validation as necessary
+func (p *PubSub) pushMsg(vals []*topicVal, src peer.ID, msg *Message) {
+	if len(vals) > 0 {
+		// validation is asynchronous and globally throttled with the throttleValidate semaphore.
+		// the purpose of the global throttle is to bound the goncurrency possible from incoming
+		// network traffic; each validator also has an individual throttle to preclude
+		// slow (or faulty) validators from starving other topics; see validate below.
+		select {
+		case p.validateThrottle <- struct{}{}:
+			go func() {
+				p.validate(vals, src, msg)
+				<-p.validateThrottle
+			}()
+		default:
+			log.Warningf("message validation throttled; dropping message from %s", src)
+		}
+		return
+	}
+
+	p.maybePublishMessage(src, msg.Message)
+}
+
+// validate performs validation and only sends the message if all validators succeed
+func (p *PubSub) validate(vals []*topicVal, src peer.ID, msg *Message) {
+	ctx, cancel := context.WithCancel(p.ctx)
+	defer cancel()
+
+	rch := make(chan bool, len(vals))
+	rcount := 0
+	throttle := false
+
+loop:
+	for _, val := range vals {
+		rcount++
+
+		select {
+		case val.validateThrottle <- struct{}{}:
+			go func(val *topicVal) {
+				rch <- val.validateMsg(ctx, msg)
+				<-val.validateThrottle
+			}(val)
+
+		default:
+			log.Debugf("validation throttled for topic %s", val.topic)
+			throttle = true
+			break loop
+		}
+	}
+
+	if throttle {
+		log.Warningf("message validation throttled; dropping message from %s", src)
+		return
+	}
+
+	for i := 0; i < rcount; i++ {
+		valid := <-rch
+		if !valid {
+			log.Warningf("message validation failed; dropping message from %s", src)
+			return
+		}
+	}
+
+	// all validators were successful, send the message
+	p.sendMsg <- &sendReq{
+		from: src,
+		msg:  msg,
+	}
+}
+
 func (p *PubSub) maybePublishMessage(from peer.ID, pmsg *pb.Message) {
 	id := msgID(pmsg)
 	if p.seenMessage(id) {
@@ -348,7 +463,7 @@ func (p *PubSub) publishMessage(from peer.ID, msg *pb.Message) error {
 			continue
 		}
 
-		for p, _ := range tmap {
+		for p := range tmap {
 			tosend[p] = struct{}{}
 		}
 	}
@@ -375,20 +490,38 @@ func (p *PubSub) publishMessage(from peer.ID, msg *pb.Message) error {
 	return nil
 }
 
+// getValidators returns all validators that apply to a given message
+func (p *PubSub) getValidators(msg *Message) []*topicVal {
+	var vals []*topicVal
+
+	for _, topic := range msg.GetTopicIDs() {
+		val, ok := p.topicVals[topic]
+		if !ok {
+			continue
+		}
+
+		vals = append(vals, val)
+	}
+
+	return vals
+}
+
 type addSubReq struct {
-	topic string
-	resp  chan *Subscription
+	sub  *Subscription
+	resp chan *Subscription
 }
 
+type SubOpt func(sub *Subscription) error
+
 // Subscribe returns a new Subscription for the given topic
-func (p *PubSub) Subscribe(topic string) (*Subscription, error) {
+func (p *PubSub) Subscribe(topic string, opts ...SubOpt) (*Subscription, error) {
 	td := pb.TopicDescriptor{Name: &topic}
 
-	return p.SubscribeByTopicDescriptor(&td)
+	return p.SubscribeByTopicDescriptor(&td, opts...)
 }
 
 // SubscribeByTopicDescriptor lets you subscribe a topic using a pb.TopicDescriptor
-func (p *PubSub) SubscribeByTopicDescriptor(td *pb.TopicDescriptor) (*Subscription, error) {
+func (p *PubSub) SubscribeByTopicDescriptor(td *pb.TopicDescriptor, opts ...SubOpt) (*Subscription, error) {
 	if td.GetAuth().GetMode() != pb.TopicDescriptor_AuthOpts_NONE {
 		return nil, fmt.Errorf("auth mode not yet supported")
 	}
@@ -397,10 +530,21 @@ func (p *PubSub) SubscribeByTopicDescriptor(td *pb.TopicDescriptor) (*Subscripti
 		return nil, fmt.Errorf("encryption mode not yet supported")
 	}
 
+	sub := &Subscription{
+		topic: td.GetName(),
+	}
+
+	for _, opt := range opts {
+		err := opt(sub)
+		if err != nil {
+			return nil, err
+		}
+	}
+
 	out := make(chan *Subscription, 1)
 	p.addSub <- &addSubReq{
-		topic: td.GetName(),
-		resp:  out,
+		sub:  sub,
+		resp: out,
 	}
 
 	return <-out, nil
@@ -439,6 +583,12 @@ type listPeerReq struct {
 	topic string
 }
 
+// sendReq is a request to call maybePublishMessage. It is issued after the subscription verification is done.
+type sendReq struct {
+	from peer.ID
+	msg  *Message
+}
+
 // ListPeers returns a list of peers we are connected to.
 func (p *PubSub) ListPeers(topic string) []peer.ID {
 	out := make(chan []peer.ID)
@@ -448,3 +598,100 @@ func (p *PubSub) ListPeers(topic string) []peer.ID {
 	}
 	return <-out
 }
+
+// per topic validators
+type addValReq struct {
+	topic    string
+	validate Validator
+	timeout  time.Duration
+	throttle int
+	resp     chan error
+}
+
+type topicVal struct {
+	topic            string
+	validate         Validator
+	validateTimeout  time.Duration
+	validateThrottle chan struct{}
+}
+
+// Validator is a function that validates a message
+type Validator func(context.Context, *Message) bool
+
+// ValidatorOpt is an option for RegisterTopicValidator
+type ValidatorOpt func(addVal *addValReq) error
+
+// WithValidatorTimeout is an option that sets the topic validator timeout
+func WithValidatorTimeout(timeout time.Duration) ValidatorOpt {
+	return func(addVal *addValReq) error {
+		addVal.timeout = timeout
+		return nil
+	}
+}
+
+// WithValidatorConcurrency is an option that sets topic validator throttle
+func WithValidatorConcurrency(n int) ValidatorOpt {
+	return func(addVal *addValReq) error {
+		addVal.throttle = n
+		return nil
+	}
+}
+
+// RegisterTopicValidator registers a validator for topic
+func (p *PubSub) RegisterTopicValidator(topic string, val Validator, opts ...ValidatorOpt) error {
+	addVal := &addValReq{
+		topic:    topic,
+		validate: val,
+		resp:     make(chan error, 1),
+	}
+
+	for _, opt := range opts {
+		err := opt(addVal)
+		if err != nil {
+			return err
+		}
+	}
+
+	p.addVal <- addVal
+	return <-addVal.resp
+}
+
+func (ps *PubSub) addValidator(req *addValReq) {
+	topic := req.topic
+
+	_, ok := ps.topicVals[topic]
+	if ok {
+		req.resp <- fmt.Errorf("Duplicate validator for topic %s", topic)
+		return
+	}
+
+	val := &topicVal{
+		topic:            topic,
+		validate:         req.validate,
+		validateTimeout:  defaultValidateTimeout,
+		validateThrottle: make(chan struct{}, defaultValidateConcurrency),
+	}
+
+	if req.timeout > 0 {
+		val.validateTimeout = req.timeout
+	}
+
+	if req.throttle > 0 {
+		val.validateThrottle = make(chan struct{}, req.throttle)
+	}
+
+	ps.topicVals[topic] = val
+	req.resp <- nil
+}
+
+func (val *topicVal) validateMsg(ctx context.Context, msg *Message) bool {
+	vctx, cancel := context.WithTimeout(ctx, val.validateTimeout)
+	defer cancel()
+
+	valid := val.validate(vctx, msg)
+	if !valid {
+		log.Debugf("validation failed for topic %s", val.topic)
+	}
+
+	return valid
+}
diff --git a/floodsub_test.go b/floodsub_test.go
index 28c10781..bf5ac80b 100644
--- a/floodsub_test.go
+++ b/floodsub_test.go
@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"math/rand"
 	"sort"
+	"sync"
 	"testing"
 	"time"
 
@@ -80,10 +81,14 @@ func connectAll(t *testing.T, hosts []host.Host) {
 	}
 }
 
-func getPubsubs(ctx context.Context, hs []host.Host) []*PubSub {
+func getPubsubs(ctx context.Context, hs []host.Host, opts ...Option) []*PubSub {
 	var psubs []*PubSub
 	for _, h := range hs {
-		psubs = append(psubs, NewFloodSub(ctx, h))
+		ps, err := NewFloodSub(ctx, h, opts...)
+		if err != nil {
+			panic(err)
+		}
+		psubs = append(psubs, ps)
 	}
 	return psubs
 }
@@ -289,11 +294,14 @@ func TestSelfReceive(t *testing.T) {
 
 	host := getNetHosts(t, ctx, 1)[0]
 
-	psub := NewFloodSub(ctx, host)
+	psub, err := NewFloodSub(ctx, host)
+	if err != nil {
+		t.Fatal(err)
+	}
 
 	msg := []byte("hello world")
 
-	err := psub.Publish("foobar", msg)
+	err = psub.Publish("foobar", msg)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -323,14 +331,181 @@ func TestOneToOne(t *testing.T) {
 
 	connect(t, hosts[0], hosts[1])
 
-	ch, err := psubs[1].Subscribe("foobar")
+	sub, err := psubs[1].Subscribe("foobar")
 	if err != nil {
 		t.Fatal(err)
 	}
 
 	time.Sleep(time.Millisecond * 50)
 
-	checkMessageRouting(t, "foobar", psubs, []*Subscription{ch})
+	checkMessageRouting(t, "foobar", psubs, []*Subscription{sub})
+}
+
+func TestValidate(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	hosts := getNetHosts(t, ctx, 2)
+	psubs := getPubsubs(ctx, hosts)
+
+	connect(t, hosts[0], hosts[1])
+	topic := "foobar"
+
+	err := psubs[1].RegisterTopicValidator(topic, func(ctx context.Context, msg *Message) bool {
+		return !bytes.Contains(msg.Data, []byte("illegal"))
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	sub, err := psubs[1].Subscribe(topic)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	time.Sleep(time.Millisecond * 50)
+
+	msgs := []struct {
+		msg       []byte
+		validates bool
+	}{
+		{msg: []byte("this is a legal message"), validates: true},
+		{msg: []byte("there also is nothing controversial about this message"), validates: true},
+		{msg: []byte("openly illegal content will be censored"), validates: false},
+		{msg: []byte("but subversive actors will use leetspeek to spread 1ll3g4l content"), validates: true},
+	}
+
+	for _, tc := range msgs {
+		for _, p := range psubs {
+			err := p.Publish(topic, tc.msg)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			select {
+			case msg := <-sub.ch:
+				if !tc.validates {
+					t.Log(msg)
+					t.Error("expected message validation to filter out the message")
+				}
+			case <-time.After(333 * time.Millisecond):
+				if tc.validates {
+					t.Error("expected message validation to accept the message")
+				}
+			}
+		}
+	}
+}
+
+func TestValidateOverload(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	type msg struct {
+		msg       []byte
+		validates bool
+	}
+
+	tcs := []struct {
+		msgs []msg
+
+		maxConcurrency int
+	}{
+		{
+			maxConcurrency: 10,
+			msgs: []msg{
+				{msg: []byte("this is a legal message"), validates: true},
+				{msg: []byte("but subversive actors will use leetspeek to spread 1ll3g4l content"), validates: true},
+				{msg: []byte("there also is nothing controversial about this message"), validates: true},
+				{msg: []byte("also fine"), validates: true},
+				{msg: []byte("still, all good"), validates: true},
+				{msg: []byte("this is getting boring"), validates: true},
+				{msg: []byte("foo"), validates: true},
+				{msg: []byte("foobar"), validates: true},
+				{msg: []byte("foofoo"), validates: true},
+				{msg: []byte("barfoo"), validates: true},
+				{msg: []byte("oh no!"), validates: false},
+			},
+		},
+		{
+			maxConcurrency: 2,
+			msgs: []msg{
+				{msg: []byte("this is a legal message"), validates: true},
+				{msg: []byte("but subversive actors will use leetspeek to spread 1ll3g4l content"), validates: true},
+				{msg: []byte("oh no!"), validates: false},
+			},
+		},
+	}
+
+	for _, tc := range tcs {
+
+		hosts := getNetHosts(t, ctx, 2)
+		psubs := getPubsubs(ctx, hosts)
+
+		connect(t, hosts[0], hosts[1])
+		topic := "foobar"
+
+		block := make(chan struct{})
+
+		err := psubs[1].RegisterTopicValidator(topic,
+			func(ctx context.Context, msg *Message) bool {
+				<-block
+				return true
+			},
+			WithValidatorConcurrency(tc.maxConcurrency))
+
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		sub, err := psubs[1].Subscribe(topic)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		time.Sleep(time.Millisecond * 50)
+
+		if len(tc.msgs) != tc.maxConcurrency+1 {
+			t.Fatalf("expected number of messages sent to be maxConcurrency+1. Got %d, expected %d", len(tc.msgs), tc.maxConcurrency+1)
+		}
+
+		p := psubs[0]
+
+		var wg sync.WaitGroup
+		wg.Add(1)
+		go func() {
+			for _, tmsg := range tc.msgs {
+				select {
+				case msg := <-sub.ch:
+					if !tmsg.validates {
+						t.Log(msg)
+						t.Error("expected message validation to drop the message because all validator goroutines are taken")
+					}
+				case <-time.After(333 * time.Millisecond):
+					if tmsg.validates {
+						t.Error("expected message validation to accept the message")
+					}
+				}
+			}
+			wg.Done()
+		}()
+
+		for i, tmsg := range tc.msgs {
+			err := p.Publish(topic, tmsg.msg)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			// wait a bit to let pubsub's internal state machine start validating the message
+			time.Sleep(10 * time.Millisecond)
+
+			// unblock validator goroutines after we sent one too many
+			if i == len(tc.msgs)-1 {
+				close(block)
+			}
+		}
+		wg.Wait()
+	}
 }
 
 func assertPeerLists(t *testing.T, hosts []host.Host, ps *PubSub, has ...int) {
@@ -414,7 +589,10 @@ func TestSubReporting(t *testing.T) {
 	defer cancel()
 
 	host := getNetHosts(t, ctx, 1)[0]
-	psub := NewFloodSub(ctx, host)
+	psub, err := NewFloodSub(ctx, host)
+	if err != nil {
+		t.Fatal(err)
+	}
 
 	fooSub, err := psub.Subscribe("foo")
 	if err != nil {