diff --git a/score.go b/score.go
index 9ecfa54..393b982 100644
--- a/score.go
+++ b/score.go
@@ -1,6 +1,7 @@
 package pubsub
 
 import (
+	"sync"
 	"time"
 
 	"github.com/libp2p/go-libp2p-core/peer"
@@ -11,6 +12,10 @@ type PeerScoreParams struct {
 	// Score parameters per topic.
 	Topics map[string]*TopicScoreParams
 
+	// P5: Application-specific peer scoring
+	AppSpecificScore  func(p peer.ID) float64
+	AppSpecificWeight float64
+
 	// P6: IP-colocation factor.
 	// The parameter has an associated counter which counts the number of peers with the same IP.
 	// If the number of peers in the same IP exceeds IPColocationFactorThreshold, then the value
@@ -22,10 +27,6 @@ type PeerScoreParams struct {
 	IPColocationFactorWeight    float64
 	IPColocationFactorThreshold int
 
-	// Application-specific peer scoring
-	AppSpecificScore  func(p peer.ID) float64
-	AppSpecificWeight float64
-
 	// the decay interval for parameter counters.
 	DecayInterval time.Duration
 
@@ -46,7 +47,7 @@ type TopicScoreParams struct {
 	// The weight of the parameter MUST be positive.
 	TimeInMeshWeight  float64
 	TimeInMeshQuantum time.Duration
-	TimeInMeshCap     int
+	TimeInMeshCap     float64
 
 	// P2: first message deliveries
 	// This is the number of message deliveries in the topic.
@@ -54,7 +55,7 @@ type TopicScoreParams struct {
 	// by FirstMessageDeliveriesCap.
 	// The weight of the parameter MUST be positive.
 	FirstMessageDeliveriesWeight, FirstMessageDeliveriesDecay float64
-	FirstMessageDeliveriesCap                                 int
+	FirstMessageDeliveriesCap                                 float64
 
 	// P3: mesh message deliveries
 	// This is the number of message deliveries in the mesh, within the MeshMessageDeliveriesLatency of
@@ -66,7 +67,7 @@ type TopicScoreParams struct {
 	// The penalty is only activated after MeshMessageDeliveriesActivation time in the mesh.
 	// The weight of the parameter MUST be negative.
 	MeshMessageDeliveriesWeight, MeshMessageDeliveriesDecay       float64
-	MeshMessageDeliveriesThreshold                                int
+	MeshMessageDeliveriesThreshold                                float64
 	MeshMessageDeliveriesLatency, MeshMessageDeliveriesActivation time.Duration
 
 	// P4: invalid messages
@@ -76,7 +77,55 @@ type TopicScoreParams struct {
 	InvalidMessageDeliveriesWeight, InvalidMessageDeliveriesDecay float64
 }
 
+type peerStats struct {
+	// true if the peer is currently connected
+	connected bool
+
+	// expiration time of the score stats for disconnected peers
+	expire time.Time
+
+	// per topc stats
+	topics map[string]*topicStats
+
+	// IP tracking; store as string for easy processing
+	ips []string
+}
+
+type topicStats struct {
+	// true if the peer is in the mesh
+	inMesh bool
+
+	// time when the peer was (last) GRAFTed; valid only when in mesh
+	graftTime time.Time
+
+	// time in mesh (updated during refresh/decay to avoid calling gettimeofday on
+	// every score invocation)
+	meshTime time.Duration
+
+	// first message deliveries
+	firstMessageDeliveries float64
+
+	// mesh message deliveries
+	meshMessageDeliveries float64
+
+	// true if the peer has been enough time in the mesh to activate mess message deliveries
+	meshMessageDeliveriesActive bool
+
+	// invalid message counter
+	invalidMessageDeliveries float64
+}
+
 type peerScore struct {
+	sync.Mutex
+
+	// the score parameters
+	params *PeerScoreParams
+
+	// per peer stats for score calculation
+	peerStats map[peer.ID]*peerStats
+
+	// IP colocation tracking
+	peerIPs map[string]peer.ID
 }
 
 func newPeerScore(params *PeerScoreParams) *peerScore {
@@ -89,7 +138,70 @@ func (ps *peerScore) Start(gs *GossipSubRouter) {
 }
 
 func (ps *peerScore) Score(p peer.ID) float64 {
-	return 0
+	ps.Lock()
+	defer ps.Unlock()
+
+	pstats, ok := ps.peerStats[p]
+	if !ok {
+		return 0
+	}
+
+	var score float64
+
+	// topic scores
+	for topic, tstats := range pstats.topics {
+		// the topic score
+		var topicScore float64
+
+		// the topic parameters
+		topicParams, ok := ps.params.Topics[topic]
+		if !ok {
+			// we are not scoring this topic
+			continue
+		}
+
+		// P1: time in Mesh
+		if tstats.inMesh {
+			p1 := float64(tstats.meshTime / topicParams.TimeInMeshQuantum)
+			topicScore += p1 * topicParams.TimeInMeshWeight
+		}
+
+		// P2: first message deliveries
+		p2 := tstats.firstMessageDeliveries
+		topicScore += p2 * topicParams.FirstMessageDeliveriesWeight
+
+		// P3: mesh message deliveries
+		if tstats.meshMessageDeliveriesActive {
+			if tstats.meshMessageDeliveries < topicParams.MeshMessageDeliveriesThreshold {
+				deficit := topicParams.MeshMessageDeliveriesThreshold - tstats.meshMessageDeliveries
+				p3 := deficit * deficit
+				topicScore += p3 * topicParams.MeshMessageDeliveriesWeight
+			}
+		}
+
+		// P4: invalid messages
+		p4 := tstats.invalidMessageDeliveries
+		topicScore += p4 * topicParams.InvalidMessageDeliveriesWeight
+
+		// update score, mixing with topic weight
+		score += topicScore * topicParams.TopicWeight
+	}
+
+	// P5: application-specific score
+	p5 := ps.params.AppSpecificScore(p)
+	score += p5 * ps.params.AppSpecificWeight
+
+	// P6: IP collocation factor
+	for _, ip := range pstats.ips {
+		peersInIP := len(ps.peerIPs[ip])
+		if peersInIP > ps.params.IPColocationFactorThreshold {
+			surpluss := float64(peersInIP - ps.params.IPColocationFactorThreshold)
+			p6 := surpluss * surpluss
+			score += p6 * ps.params.IPColocationFactorWeight
+		}
+	}
+
+	return score
 }
 
 // tracer interface