diff --git a/bitswap.go b/bitswap.go
index 9afe5d2750c0e39a26b32985f500953cdeb1a955..8af786a80dcb481eb131494feb3ab8ece3acc925 100644
--- a/bitswap.go
+++ b/bitswap.go
@@ -11,6 +11,7 @@ import (
 
 	delay "github.com/ipfs/go-ipfs-delay"
 
+	deciface "github.com/ipfs/go-bitswap/decision"
 	bsbpm "github.com/ipfs/go-bitswap/internal/blockpresencemanager"
 	decision "github.com/ipfs/go-bitswap/internal/decision"
 	bsgetter "github.com/ipfs/go-bitswap/internal/getter"
@@ -95,6 +96,13 @@ func SetSendDontHaves(send bool) Option {
 	}
 }
 
+// Configures the engine to use the given score decision logic.
+func WithScoreLedger(scoreLedger deciface.ScoreLedger) Option {
+	return func(bs *Bitswap) {
+		bs.engine.UseScoreLedger(scoreLedger)
+	}
+}
+
 // New initializes a BitSwap instance that communicates over the provided
 // BitSwapNetwork. This function registers the returned instance as the network
 // delegate. Runs until context is cancelled or bitswap.Close is called.
diff --git a/bitswap_test.go b/bitswap_test.go
index ba89e038d19a97265b708522b9fdf83ce5f7cfce..b95faa30d5d4f6f45857f2c1ff75b7ea5f1af3b3 100644
--- a/bitswap_test.go
+++ b/bitswap_test.go
@@ -9,11 +9,12 @@ import (
 	"time"
 
 	bitswap "github.com/ipfs/go-bitswap"
+	deciface "github.com/ipfs/go-bitswap/decision"
 	decision "github.com/ipfs/go-bitswap/internal/decision"
 	bssession "github.com/ipfs/go-bitswap/internal/session"
+	"github.com/ipfs/go-bitswap/message"
 	testinstance "github.com/ipfs/go-bitswap/testinstance"
 	tn "github.com/ipfs/go-bitswap/testnet"
-	"github.com/ipfs/go-bitswap/message"
 	blocks "github.com/ipfs/go-block-format"
 	cid "github.com/ipfs/go-cid"
 	detectrace "github.com/ipfs/go-detect-race"
@@ -803,3 +804,59 @@ func TestBitswapLedgerTwoWay(t *testing.T) {
 		}
 	}
 }
+
+type testingScoreLedger struct {
+	scorePeer deciface.ScorePeerFunc
+	started   chan struct{}
+	closed    chan struct{}
+}
+
+func newTestingScoreLedger() *testingScoreLedger {
+	return &testingScoreLedger{
+		nil,
+		make(chan struct{}),
+		make(chan struct{}),
+	}
+}
+
+func (tsl *testingScoreLedger) GetReceipt(p peer.ID) *deciface.Receipt {
+	return nil
+}
+func (tsl *testingScoreLedger) AddToSentBytes(p peer.ID, n int)     {}
+func (tsl *testingScoreLedger) AddToReceivedBytes(p peer.ID, n int) {}
+func (tsl *testingScoreLedger) PeerConnected(p peer.ID)             {}
+func (tsl *testingScoreLedger) PeerDisconnected(p peer.ID)          {}
+func (tsl *testingScoreLedger) Start(scorePeer deciface.ScorePeerFunc) {
+	tsl.scorePeer = scorePeer
+	close(tsl.started)
+}
+func (tsl *testingScoreLedger) Stop() {
+	close(tsl.closed)
+}
+
+// Tests start and stop of a custom decision logic
+func TestWithScoreLedger(t *testing.T) {
+	tsl := newTestingScoreLedger()
+	net := tn.VirtualNetwork(mockrouting.NewServer(), delay.Fixed(kNetworkDelay))
+	bsOpts := []bitswap.Option{bitswap.WithScoreLedger(tsl)}
+	ig := testinstance.NewTestInstanceGenerator(net, nil, bsOpts)
+	defer ig.Close()
+	i := ig.Next()
+	defer i.Exchange.Close()
+
+	select {
+	case <-tsl.started:
+		if tsl.scorePeer == nil {
+			t.Fatal("Expected the score function to be initialized")
+		}
+	case <-time.After(time.Second * 5):
+		t.Fatal("Expected the score ledger to be started within 5s")
+	}
+
+	i.Exchange.Close()
+	select {
+	case <-tsl.closed:
+	case <-time.After(time.Second * 5):
+		t.Fatal("Expected the score ledger to be closed within 5s")
+	}
+}
diff --git a/decision/decision.go b/decision/decision.go
index 8dd310f69ea8e09421607f24d3bf0b734d68c8d1..4afc463ec08db9bb344cc758729c5b3cc4f2134a 100644
--- a/decision/decision.go
+++ b/decision/decision.go
@@ -2,5 +2,11 @@ package decision
 
 import intdec "github.com/ipfs/go-bitswap/internal/decision"
 
-// Expose type externally
+// Expose Receipt externally
 type Receipt = intdec.Receipt
+
+// Expose ScoreLedger externally
+type ScoreLedger = intdec.ScoreLedger
+
+// Expose ScorePeerFunc externally
+type ScorePeerFunc = intdec.ScorePeerFunc
diff --git a/internal/decision/engine.go b/internal/decision/engine.go
index 2a6dc60f6d1744f67209a408ea2ab11862f49705..28584fb100e7468c3a84114c1962c32cd9f92d71 100644
--- a/internal/decision/engine.go
+++ b/internal/decision/engine.go
@@ -70,25 +70,6 @@ const (
 	// on their behalf.
 	queuedTagWeight = 10
 
-	// the alpha for the EWMA used to track short term usefulness
-	shortTermAlpha = 0.5
-
-	// the alpha for the EWMA used to track long term usefulness
-	longTermAlpha = 0.05
-
-	// how frequently the engine should sample usefulness. Peers that
-	// interact every shortTerm time period are considered "active".
-	shortTerm = 10 * time.Second
-
-	// long term ratio defines what "long term" means in terms of the
-	// shortTerm duration. Peers that interact once every longTermRatio are
-	// considered useful over the long term.
-	longTermRatio = 10
-
-	// long/short term scores for tagging peers
-	longTermScore  = 10 // this is a high tag but it grows _very_ slowly.
-	shortTermScore = 10 // this is a high tag but it'll go away quickly if we aren't using the peer.
-
 	// maxBlockSizeReplaceHasWithBlock is the maximum size of the block in
 	// bytes up to which we will replace a want-have with a want-block
 	maxBlockSizeReplaceHasWithBlock = 1024
@@ -119,6 +100,29 @@ type PeerTagger interface {
 	UntagPeer(p peer.ID, tag string)
 }
 
+// Assigns a specific score to a peer
+type ScorePeerFunc func(peer.ID, int)
+
+// ScoreLedger is an external ledger dealing with peer scores.
+type ScoreLedger interface {
+	// Returns aggregated data communication with a given peer.
+	GetReceipt(p peer.ID) *Receipt
+	// Increments the sent counter for the given peer.
+	AddToSentBytes(p peer.ID, n int)
+	// Increments the received counter for the given peer.
+	AddToReceivedBytes(p peer.ID, n int)
+	// PeerConnected should be called when a new peer connects,
+	// meaning the ledger should open accounting.
+	PeerConnected(p peer.ID)
+	// PeerDisconnected should be called when a peer disconnects to
+	// clean up the accounting.
+	PeerDisconnected(p peer.ID)
+	// Starts the ledger sampling process.
+	Start(scorePeer ScorePeerFunc)
+	// Stops the sampling process.
+	Stop()
+}
+
 // Engine manages sending requested blocks to peers.
 type Engine struct {
 	// peerRequestQueue is a priority queue of requests received from peers.
@@ -145,9 +149,12 @@ type Engine struct {
 
 	lock sync.RWMutex // protects the fields immediatly below
 
-	// ledgerMap lists Ledgers by their Partner key.
+	// ledgerMap lists block-related Ledgers by their Partner key.
 	ledgerMap map[peer.ID]*ledger
 
+	// an external ledger dealing with peer scores
+	scoreLedger ScoreLedger
+
 	ticker *time.Ticker
 
 	taskWorkerLock  sync.Mutex
@@ -157,11 +164,6 @@ type Engine struct {
 	// bytes up to which we will replace a want-have with a want-block
 	maxBlockSizeReplaceHasWithBlock int
 
-	// how frequently the engine should sample peer usefulness
-	peerSampleInterval time.Duration
-	// used by the tests to detect when a sample is taken
-	sampleCh chan struct{}
-
 	sendDontHaves bool
 
 	self peer.ID
@@ -169,23 +171,22 @@ type Engine struct {
 
 // NewEngine creates a new block sending engine for the given block store
 func NewEngine(ctx context.Context, bs bstore.Blockstore, peerTagger PeerTagger, self peer.ID) *Engine {
-	return newEngine(ctx, bs, peerTagger, self, maxBlockSizeReplaceHasWithBlock, shortTerm, nil)
+	return newEngine(ctx, bs, peerTagger, self, maxBlockSizeReplaceHasWithBlock, nil)
 }
 
 // This constructor is used by the tests
 func newEngine(ctx context.Context, bs bstore.Blockstore, peerTagger PeerTagger, self peer.ID,
-	maxReplaceSize int, peerSampleInterval time.Duration, sampleCh chan struct{}) *Engine {
+	maxReplaceSize int, scoreLedger ScoreLedger) *Engine {
 
 	e := &Engine{
 		ledgerMap:                       make(map[peer.ID]*ledger),
+		scoreLedger:                     scoreLedger,
 		bsm:                             newBlockstoreManager(ctx, bs, blockstoreWorkerCount),
 		peerTagger:                      peerTagger,
 		outbox:                          make(chan (<-chan *Envelope), outboxChanBuffer),
 		workSignal:                      make(chan struct{}, 1),
 		ticker:                          time.NewTicker(time.Millisecond * 100),
 		maxBlockSizeReplaceHasWithBlock: maxReplaceSize,
-		peerSampleInterval:              peerSampleInterval,
-		sampleCh:                        sampleCh,
 		taskWorkerCount:                 taskWorkerCount,
 		sendDontHaves:                   true,
 		self:                            self,
@@ -210,11 +211,37 @@ func (e *Engine) SetSendDontHaves(send bool) {
 	e.sendDontHaves = send
 }
 
+// Sets the scoreLedger to the given implementation. Should be called
+// before StartWorkers().
+func (e *Engine) UseScoreLedger(scoreLedger ScoreLedger) {
+	e.scoreLedger = scoreLedger
+}
+
+// Starts the score ledger. Before start the function checks and,
+// if it is unset, initializes the scoreLedger with the default
+// implementation.
+func (e *Engine) startScoreLedger(px process.Process) {
+	if e.scoreLedger == nil {
+		e.scoreLedger = NewDefaultScoreLedger()
+	}
+	e.scoreLedger.Start(func(p peer.ID, score int) {
+		if score == 0 {
+			e.peerTagger.UntagPeer(p, e.tagUseful)
+		} else {
+			e.peerTagger.TagPeer(p, e.tagUseful, score)
+		}
+	})
+	px.Go(func(ppx process.Process) {
+		<-ppx.Closing()
+		e.scoreLedger.Stop()
+	})
+}
+
 // Start up workers to handle requests from other nodes for the data on this node
 func (e *Engine) StartWorkers(ctx context.Context, px process.Process) {
 	// Start up blockstore manager
 	e.bsm.start(px)
-	px.Go(e.scoreWorker)
+	e.startScoreLedger(px)
 
 	for i := 0; i < e.taskWorkerCount; i++ {
 		px.Go(func(px process.Process) {
@@ -223,109 +250,6 @@ func (e *Engine) StartWorkers(ctx context.Context, px process.Process) {
 	}
 }
 
-// scoreWorker keeps track of how "useful" our peers are, updating scores in the
-// connection manager.
-//
-// It does this by tracking two scores: short-term usefulness and long-term
-// usefulness. Short-term usefulness is sampled frequently and highly weights
-// new observations. Long-term usefulness is sampled less frequently and highly
-// weights on long-term trends.
-//
-// In practice, we do this by keeping two EWMAs. If we see an interaction
-// within the sampling period, we record the score, otherwise, we record a 0.
-// The short-term one has a high alpha and is sampled every shortTerm period.
-// The long-term one has a low alpha and is sampled every
-// longTermRatio*shortTerm period.
-//
-// To calculate the final score, we sum the short-term and long-term scores then
-// adjust it ±25% based on our debt ratio. Peers that have historically been
-// more useful to us than we are to them get the highest score.
-func (e *Engine) scoreWorker(px process.Process) {
-	ticker := time.NewTicker(e.peerSampleInterval)
-	defer ticker.Stop()
-
-	type update struct {
-		peer  peer.ID
-		score int
-	}
-	var (
-		lastShortUpdate, lastLongUpdate time.Time
-		updates                         []update
-	)
-
-	for i := 0; ; i = (i + 1) % longTermRatio {
-		var now time.Time
-		select {
-		case now = <-ticker.C:
-		case <-px.Closing():
-			return
-		}
-
-		// The long term update ticks every `longTermRatio` short
-		// intervals.
-		updateLong := i == 0
-
-		e.lock.Lock()
-		for _, ledger := range e.ledgerMap {
-			ledger.lk.Lock()
-
-			// Update the short-term score.
-			if ledger.lastExchange.After(lastShortUpdate) {
-				ledger.shortScore = ewma(ledger.shortScore, shortTermScore, shortTermAlpha)
-			} else {
-				ledger.shortScore = ewma(ledger.shortScore, 0, shortTermAlpha)
-			}
-
-			// Update the long-term score.
-			if updateLong {
-				if ledger.lastExchange.After(lastLongUpdate) {
-					ledger.longScore = ewma(ledger.longScore, longTermScore, longTermAlpha)
-				} else {
-					ledger.longScore = ewma(ledger.longScore, 0, longTermAlpha)
-				}
-			}
-
-			// Calculate the new score.
-			//
-			// The accounting score adjustment prefers peers _we_
-			// need over peers that need us. This doesn't help with
-			// leeching.
-			score := int((ledger.shortScore + ledger.longScore) * ((ledger.Accounting.Score())*.5 + .75))
-
-			// Avoid updating the connection manager unless there's a change. This can be expensive.
-			if ledger.score != score {
-				// put these in a list so we can perform the updates outside _global_ the lock.
-				updates = append(updates, update{ledger.Partner, score})
-				ledger.score = score
-			}
-			ledger.lk.Unlock()
-		}
-		e.lock.Unlock()
-
-		// record the times.
-		lastShortUpdate = now
-		if updateLong {
-			lastLongUpdate = now
-		}
-
-		// apply the updates
-		for _, update := range updates {
-			if update.score == 0 {
-				e.peerTagger.UntagPeer(update.peer, e.tagUseful)
-			} else {
-				e.peerTagger.TagPeer(update.peer, e.tagUseful, update.score)
-			}
-		}
-		// Keep the memory. It's not much and it saves us from having to allocate.
-		updates = updates[:0]
-
-		// Used by the tests
-		if e.sampleCh != nil {
-			e.sampleCh <- struct{}{}
-		}
-	}
-}
-
 func (e *Engine) onPeerAdded(p peer.ID) {
 	e.peerTagger.TagPeer(p, e.tagQueued, queuedTagWeight)
 }
@@ -347,21 +271,9 @@ func (e *Engine) WantlistForPeer(p peer.ID) []wl.Entry {
 	return entries
 }
 
-// LedgerForPeer returns aggregated data about blocks swapped and communication
-// with a given peer.
+// LedgerForPeer returns aggregated data communication with a given peer.
 func (e *Engine) LedgerForPeer(p peer.ID) *Receipt {
-	ledger := e.findOrCreate(p)
-
-	ledger.lk.Lock()
-	defer ledger.lk.Unlock()
-
-	return &Receipt{
-		Peer:      ledger.Partner.String(),
-		Value:     ledger.Accounting.Value(),
-		Sent:      ledger.Accounting.BytesSent,
-		Recv:      ledger.Accounting.BytesRecv,
-		Exchanged: ledger.ExchangeCount(),
-	}
+	return e.scoreLedger.GetReceipt(p)
 }
 
 // Each taskWorker pulls items off the request queue up to the maximum size
@@ -671,7 +583,7 @@ func (e *Engine) ReceiveFrom(from peer.ID, blks []blocks.Block, haves []cid.Cid)
 		// Record how many bytes were received in the ledger
 		for _, blk := range blks {
 			log.Debugw("Bitswap engine <- block", "local", e.self, "from", from, "cid", blk.Cid(), "size", len(blk.RawData()))
-			l.ReceivedBytes(len(blk.RawData()))
+			e.scoreLedger.AddToReceivedBytes(l.Partner, len(blk.RawData()))
 		}
 
 		l.lk.Unlock()
@@ -741,7 +653,7 @@ func (e *Engine) MessageSent(p peer.ID, m bsmsg.BitSwapMessage) {
 
 	// Remove sent blocks from the want list for the peer
 	for _, block := range m.Blocks() {
-		l.SentBytes(len(block.RawData()))
+		e.scoreLedger.AddToSentBytes(l.Partner, len(block.RawData()))
 		l.wantList.RemoveType(block.Cid(), pb.Message_Wantlist_Block)
 	}
 
@@ -764,6 +676,8 @@ func (e *Engine) PeerConnected(p peer.ID) {
 	if !ok {
 		e.ledgerMap[p] = newLedger(p)
 	}
+
+	e.scoreLedger.PeerConnected(p)
 }
 
 // PeerDisconnected is called when a peer disconnects.
@@ -772,6 +686,8 @@ func (e *Engine) PeerDisconnected(p peer.ID) {
 	defer e.lock.Unlock()
 
 	delete(e.ledgerMap, p)
+
+	e.scoreLedger.PeerDisconnected(p)
 }
 
 // If the want is a want-have, and it's below a certain size, send the full
@@ -782,13 +698,11 @@ func (e *Engine) sendAsBlock(wantType pb.Message_Wantlist_WantType, blockSize in
 }
 
 func (e *Engine) numBytesSentTo(p peer.ID) uint64 {
-	// NB not threadsafe
-	return e.findOrCreate(p).Accounting.BytesSent
+	return e.LedgerForPeer(p).Sent
 }
 
 func (e *Engine) numBytesReceivedFrom(p peer.ID) uint64 {
-	// NB not threadsafe
-	return e.findOrCreate(p).Accounting.BytesRecv
+	return e.LedgerForPeer(p).Recv
 }
 
 // ledger lazily instantiates a ledger
diff --git a/internal/decision/engine_test.go b/internal/decision/engine_test.go
index 3cb76597377f3798b57925e9804d966ea5aa680d..3046dc0d1d5ecb28a15390816172f08c8dcad974 100644
--- a/internal/decision/engine_test.go
+++ b/internal/decision/engine_test.go
@@ -97,7 +97,7 @@ func newTestEngine(ctx context.Context, idStr string) engineSet {
 func newTestEngineWithSampling(ctx context.Context, idStr string, peerSampleInterval time.Duration, sampleCh chan struct{}) engineSet {
 	fpt := &fakePeerTagger{}
 	bs := blockstore.NewBlockstore(dssync.MutexWrap(ds.NewMapDatastore()))
-	e := newEngine(ctx, bs, fpt, "localhost", 0, peerSampleInterval, sampleCh)
+	e := newEngine(ctx, bs, fpt, "localhost", 0, NewTestScoreLedger(peerSampleInterval, sampleCh))
 	e.StartWorkers(ctx, process.WithTeardown(func() error { return nil }))
 	return engineSet{
 		Peer: peer.ID(idStr),
@@ -185,7 +185,7 @@ func peerIsPartner(p peer.ID, e *Engine) bool {
 func TestOutboxClosedWhenEngineClosed(t *testing.T) {
 	ctx := context.Background()
 	t.SkipNow() // TODO implement *Engine.Close
-	e := newEngine(ctx, blockstore.NewBlockstore(dssync.MutexWrap(ds.NewMapDatastore())), &fakePeerTagger{}, "localhost", 0, shortTerm, nil)
+	e := newEngine(ctx, blockstore.NewBlockstore(dssync.MutexWrap(ds.NewMapDatastore())), &fakePeerTagger{}, "localhost", 0, NewTestScoreLedger(shortTerm, nil))
 	e.StartWorkers(ctx, process.WithTeardown(func() error { return nil }))
 	var wg sync.WaitGroup
 	wg.Add(1)
@@ -513,7 +513,7 @@ func TestPartnerWantHaveWantBlockNonActive(t *testing.T) {
 		testCases = onlyTestCases
 	}
 
-	e := newEngine(context.Background(), bs, &fakePeerTagger{}, "localhost", 0, shortTerm, nil)
+	e := newEngine(context.Background(), bs, &fakePeerTagger{}, "localhost", 0, NewTestScoreLedger(shortTerm, nil))
 	e.StartWorkers(context.Background(), process.WithTeardown(func() error { return nil }))
 	for i, testCase := range testCases {
 		t.Logf("Test case %d:", i)
@@ -669,7 +669,7 @@ func TestPartnerWantHaveWantBlockActive(t *testing.T) {
 		testCases = onlyTestCases
 	}
 
-	e := newEngine(context.Background(), bs, &fakePeerTagger{}, "localhost", 0, shortTerm, nil)
+	e := newEngine(context.Background(), bs, &fakePeerTagger{}, "localhost", 0, NewTestScoreLedger(shortTerm, nil))
 	e.StartWorkers(context.Background(), process.WithTeardown(func() error { return nil }))
 
 	var next envChan
@@ -854,7 +854,7 @@ func TestPartnerWantsThenCancels(t *testing.T) {
 	ctx := context.Background()
 	for i := 0; i < numRounds; i++ {
 		expected := make([][]string, 0, len(testcases))
-		e := newEngine(ctx, bs, &fakePeerTagger{}, "localhost", 0, shortTerm, nil)
+		e := newEngine(ctx, bs, &fakePeerTagger{}, "localhost", 0, NewTestScoreLedger(shortTerm, nil))
 		e.StartWorkers(ctx, process.WithTeardown(func() error { return nil }))
 		for _, testcase := range testcases {
 			set := testcase[0]
@@ -879,7 +879,7 @@ func TestSendReceivedBlocksToPeersThatWantThem(t *testing.T) {
 	partner := libp2ptest.RandPeerIDFatal(t)
 	otherPeer := libp2ptest.RandPeerIDFatal(t)
 
-	e := newEngine(context.Background(), bs, &fakePeerTagger{}, "localhost", 0, shortTerm, nil)
+	e := newEngine(context.Background(), bs, &fakePeerTagger{}, "localhost", 0, NewTestScoreLedger(shortTerm, nil))
 	e.StartWorkers(context.Background(), process.WithTeardown(func() error { return nil }))
 
 	blks := testutil.GenerateBlocksOfSize(4, 8*1024)
@@ -923,7 +923,7 @@ func TestSendDontHave(t *testing.T) {
 	partner := libp2ptest.RandPeerIDFatal(t)
 	otherPeer := libp2ptest.RandPeerIDFatal(t)
 
-	e := newEngine(context.Background(), bs, &fakePeerTagger{}, "localhost", 0, shortTerm, nil)
+	e := newEngine(context.Background(), bs, &fakePeerTagger{}, "localhost", 0, NewTestScoreLedger(shortTerm, nil))
 	e.StartWorkers(context.Background(), process.WithTeardown(func() error { return nil }))
 
 	blks := testutil.GenerateBlocksOfSize(4, 8*1024)
@@ -987,7 +987,7 @@ func TestWantlistForPeer(t *testing.T) {
 	partner := libp2ptest.RandPeerIDFatal(t)
 	otherPeer := libp2ptest.RandPeerIDFatal(t)
 
-	e := newEngine(context.Background(), bs, &fakePeerTagger{}, "localhost", 0, shortTerm, nil)
+	e := newEngine(context.Background(), bs, &fakePeerTagger{}, "localhost", 0, NewTestScoreLedger(shortTerm, nil))
 	e.StartWorkers(context.Background(), process.WithTeardown(func() error { return nil }))
 
 	blks := testutil.GenerateBlocksOfSize(4, 8*1024)
diff --git a/internal/decision/ledger.go b/internal/decision/ledger.go
index 87fedc458c308ee7183c5465183ecca8835c7822..a607ff4f4e5a638d65775f68afbab21d57c215f3 100644
--- a/internal/decision/ledger.go
+++ b/internal/decision/ledger.go
@@ -2,7 +2,6 @@ package decision
 
 import (
 	"sync"
-	"time"
 
 	pb "github.com/ipfs/go-bitswap/message/pb"
 	wl "github.com/ipfs/go-bitswap/wantlist"
@@ -18,75 +17,17 @@ func newLedger(p peer.ID) *ledger {
 	}
 }
 
-// ledger stores the data exchange relationship between two peers.
-// NOT threadsafe
+// Keeps the wantlist for the partner. NOT threadsafe!
 type ledger struct {
 	// Partner is the remote Peer.
 	Partner peer.ID
 
-	// Accounting tracks bytes sent and received.
-	Accounting debtRatio
-
-	// lastExchange is the time of the last data exchange.
-	lastExchange time.Time
-
-	// These scores keep track of how useful we think this peer is. Short
-	// tracks short-term usefulness and long tracks long-term usefulness.
-	shortScore, longScore float64
-	// Score keeps track of the score used in the peer tagger. We track it
-	// here to avoid unnecessarily updating the tags in the connection manager.
-	score int
-
-	// exchangeCount is the number of exchanges with this peer
-	exchangeCount uint64
-
 	// wantList is a (bounded, small) set of keys that Partner desires.
 	wantList *wl.Wantlist
 
 	lk sync.RWMutex
 }
 
-// Receipt is a summary of the ledger for a given peer
-// collecting various pieces of aggregated data for external
-// reporting purposes.
-type Receipt struct {
-	Peer      string
-	Value     float64
-	Sent      uint64
-	Recv      uint64
-	Exchanged uint64
-}
-
-type debtRatio struct {
-	BytesSent uint64
-	BytesRecv uint64
-}
-
-// Value returns the debt ratio, sent:receive.
-func (dr *debtRatio) Value() float64 {
-	return float64(dr.BytesSent) / float64(dr.BytesRecv+1)
-}
-
-// Score returns the debt _score_ on a 0-1 scale.
-func (dr *debtRatio) Score() float64 {
-	if dr.BytesRecv == 0 {
-		return 0
-	}
-	return float64(dr.BytesRecv) / float64(dr.BytesRecv+dr.BytesSent)
-}
-
-func (l *ledger) SentBytes(n int) {
-	l.exchangeCount++
-	l.lastExchange = time.Now()
-	l.Accounting.BytesSent += uint64(n)
-}
-
-func (l *ledger) ReceivedBytes(n int) {
-	l.exchangeCount++
-	l.lastExchange = time.Now()
-	l.Accounting.BytesRecv += uint64(n)
-}
-
 func (l *ledger) Wants(k cid.Cid, priority int32, wantType pb.Message_Wantlist_WantType) {
 	log.Debugf("peer %s wants %s", l.Partner, k)
 	l.wantList.Add(k, priority, wantType)
@@ -99,7 +40,3 @@ func (l *ledger) CancelWant(k cid.Cid) bool {
 func (l *ledger) WantListContains(k cid.Cid) (wl.Entry, bool) {
 	return l.wantList.Contains(k)
 }
-
-func (l *ledger) ExchangeCount() uint64 {
-	return l.exchangeCount
-}
diff --git a/internal/decision/scoreledger.go b/internal/decision/scoreledger.go
new file mode 100644
index 0000000000000000000000000000000000000000..5ffd6bb8a7f4e141a188f9283670f1fc0f137897
--- /dev/null
+++ b/internal/decision/scoreledger.go
@@ -0,0 +1,350 @@
+package decision
+
+import (
+	"sync"
+	"time"
+
+	peer "github.com/libp2p/go-libp2p-core/peer"
+)
+
+const (
+	// the alpha for the EWMA used to track short term usefulness
+	shortTermAlpha = 0.5
+
+	// the alpha for the EWMA used to track long term usefulness
+	longTermAlpha = 0.05
+
+	// how frequently the engine should sample usefulness. Peers that
+	// interact every shortTerm time period are considered "active".
+	shortTerm = 10 * time.Second
+
+	// long term ratio defines what "long term" means in terms of the
+	// shortTerm duration. Peers that interact once every longTermRatio are
+	// considered useful over the long term.
+	longTermRatio = 10
+
+	// long/short term scores for tagging peers
+	longTermScore  = 10 // this is a high tag but it grows _very_ slowly.
+	shortTermScore = 10 // this is a high tag but it'll go away quickly if we aren't using the peer.
+)
+
+// Stores the data exchange relationship between two peers.
+type scoreledger struct {
+	// Partner is the remote Peer.
+	partner peer.ID
+
+	// tracks bytes sent...
+	bytesSent uint64
+
+	// ...and received.
+	bytesRecv uint64
+
+	// lastExchange is the time of the last data exchange.
+	lastExchange time.Time
+
+	// These scores keep track of how useful we think this peer is. Short
+	// tracks short-term usefulness and long tracks long-term usefulness.
+	shortScore, longScore float64
+
+	// Score keeps track of the score used in the peer tagger. We track it
+	// here to avoid unnecessarily updating the tags in the connection manager.
+	score int
+
+	// exchangeCount is the number of exchanges with this peer
+	exchangeCount uint64
+
+	// the record lock
+	lock sync.RWMutex
+}
+
+// Receipt is a summary of the ledger for a given peer
+// collecting various pieces of aggregated data for external
+// reporting purposes.
+type Receipt struct {
+	Peer      string
+	Value     float64
+	Sent      uint64
+	Recv      uint64
+	Exchanged uint64
+}
+
+// Increments the sent counter.
+func (l *scoreledger) AddToSentBytes(n int) {
+	l.lock.Lock()
+	defer l.lock.Unlock()
+	l.exchangeCount++
+	l.lastExchange = time.Now()
+	l.bytesSent += uint64(n)
+}
+
+// Increments the received counter.
+func (l *scoreledger) AddToReceivedBytes(n int) {
+	l.lock.Lock()
+	defer l.lock.Unlock()
+	l.exchangeCount++
+	l.lastExchange = time.Now()
+	l.bytesRecv += uint64(n)
+}
+
+// Returns the Receipt for this ledger record.
+func (l *scoreledger) Receipt() *Receipt {
+	l.lock.RLock()
+	defer l.lock.RUnlock()
+
+	return &Receipt{
+		Peer:      l.partner.String(),
+		Value:     float64(l.bytesSent) / float64(l.bytesRecv+1),
+		Sent:      l.bytesSent,
+		Recv:      l.bytesRecv,
+		Exchanged: l.exchangeCount,
+	}
+}
+
+// DefaultScoreLedger is used by Engine as the default ScoreLedger.
+type DefaultScoreLedger struct {
+	// a sample counting ticker
+	ticker *time.Ticker
+	// the score func
+	scorePeer ScorePeerFunc
+	// is closed on Close
+	closing chan struct{}
+	// protects the fields immediatly below
+	lock sync.RWMutex
+	// ledgerMap lists score ledgers by their partner key.
+	ledgerMap map[peer.ID]*scoreledger
+	// how frequently the engine should sample peer usefulness
+	peerSampleInterval time.Duration
+	// used by the tests to detect when a sample is taken
+	sampleCh chan struct{}
+}
+
+// scoreWorker keeps track of how "useful" our peers are, updating scores in the
+// connection manager.
+//
+// It does this by tracking two scores: short-term usefulness and long-term
+// usefulness. Short-term usefulness is sampled frequently and highly weights
+// new observations. Long-term usefulness is sampled less frequently and highly
+// weights on long-term trends.
+//
+// In practice, we do this by keeping two EWMAs. If we see an interaction
+// within the sampling period, we record the score, otherwise, we record a 0.
+// The short-term one has a high alpha and is sampled every shortTerm period.
+// The long-term one has a low alpha and is sampled every
+// longTermRatio*shortTerm period.
+//
+// To calculate the final score, we sum the short-term and long-term scores then
+// adjust it ±25% based on our debt ratio. Peers that have historically been
+// more useful to us than we are to them get the highest score.
+func (dsl *DefaultScoreLedger) scoreWorker() {
+	ticker := time.NewTicker(dsl.peerSampleInterval)
+	defer ticker.Stop()
+
+	type update struct {
+		peer  peer.ID
+		score int
+	}
+	var (
+		lastShortUpdate, lastLongUpdate time.Time
+		updates                         []update
+	)
+
+	for i := 0; ; i = (i + 1) % longTermRatio {
+		var now time.Time
+		select {
+		case now = <-ticker.C:
+		case <-dsl.closing:
+			return
+		}
+
+		// The long term update ticks every `longTermRatio` short
+		// intervals.
+		updateLong := i == 0
+
+		dsl.lock.Lock()
+		for _, l := range dsl.ledgerMap {
+			l.lock.Lock()
+
+			// Update the short-term score.
+			if l.lastExchange.After(lastShortUpdate) {
+				l.shortScore = ewma(l.shortScore, shortTermScore, shortTermAlpha)
+			} else {
+				l.shortScore = ewma(l.shortScore, 0, shortTermAlpha)
+			}
+
+			// Update the long-term score.
+			if updateLong {
+				if l.lastExchange.After(lastLongUpdate) {
+					l.longScore = ewma(l.longScore, longTermScore, longTermAlpha)
+				} else {
+					l.longScore = ewma(l.longScore, 0, longTermAlpha)
+				}
+			}
+
+			// Calculate the new score.
+			//
+			// The accounting score adjustment prefers peers _we_
+			// need over peers that need us. This doesn't help with
+			// leeching.
+			var lscore float64
+			if l.bytesRecv == 0 {
+				lscore = 0
+			} else {
+				lscore = float64(l.bytesRecv) / float64(l.bytesRecv+l.bytesSent)
+			}
+			score := int((l.shortScore + l.longScore) * (lscore*.5 + .75))
+
+			// Avoid updating the connection manager unless there's a change. This can be expensive.
+			if l.score != score {
+				// put these in a list so we can perform the updates outside _global_ the lock.
+				updates = append(updates, update{l.partner, score})
+				l.score = score
+			}
+			l.lock.Unlock()
+		}
+		dsl.lock.Unlock()
+
+		// record the times.
+		lastShortUpdate = now
+		if updateLong {
+			lastLongUpdate = now
+		}
+
+		// apply the updates
+		for _, update := range updates {
+			dsl.scorePeer(update.peer, update.score)
+		}
+		// Keep the memory. It's not much and it saves us from having to allocate.
+		updates = updates[:0]
+
+		// Used by the tests
+		if dsl.sampleCh != nil {
+			dsl.sampleCh <- struct{}{}
+		}
+	}
+}
+
+// Returns the score ledger for the given peer or nil if that peer
+// is not on the ledger.
+func (dsl *DefaultScoreLedger) find(p peer.ID) *scoreledger {
+	// Take a read lock (as it's less expensive) to check if we have
+	// a ledger for the peer.
+	dsl.lock.RLock()
+	l, ok := dsl.ledgerMap[p]
+	dsl.lock.RUnlock()
+	if ok {
+		return l
+	}
+	return nil
+}
+
+// Returns a new scoreledger.
+func newScoreLedger(p peer.ID) *scoreledger {
+	return &scoreledger{
+		partner: p,
+	}
+}
+
+// Lazily instantiates a ledger.
+func (dsl *DefaultScoreLedger) findOrCreate(p peer.ID) *scoreledger {
+	l := dsl.find(p)
+	if l != nil {
+		return l
+	}
+
+	// There's no ledger, so take a write lock, then check again and
+	// create the ledger if necessary.
+	dsl.lock.Lock()
+	defer dsl.lock.Unlock()
+	l, ok := dsl.ledgerMap[p]
+	if !ok {
+		l = newScoreLedger(p)
+		dsl.ledgerMap[p] = l
+	}
+	return l
+}
+
+// GetReceipt returns aggregated data communication with a given peer.
+func (dsl *DefaultScoreLedger) GetReceipt(p peer.ID) *Receipt {
+	l := dsl.find(p)
+	if l != nil {
+		return l.Receipt()
+	}
+
+	// Return a blank receipt otherwise.
+	return &Receipt{
+		Peer:      p.String(),
+		Value:     0,
+		Sent:      0,
+		Recv:      0,
+		Exchanged: 0,
+	}
+}
+
+// Starts the default ledger sampling process.
+func (dsl *DefaultScoreLedger) Start(scorePeer ScorePeerFunc) {
+	dsl.init(scorePeer)
+	go dsl.scoreWorker()
+}
+
+// Stops the sampling process.
+func (dsl *DefaultScoreLedger) Stop() {
+	close(dsl.closing)
+}
+
+// Initializes the score ledger.
+func (dsl *DefaultScoreLedger) init(scorePeer ScorePeerFunc) {
+	dsl.lock.Lock()
+	defer dsl.lock.Unlock()
+	dsl.ledgerMap = make(map[peer.ID]*scoreledger)
+	dsl.scorePeer = scorePeer
+}
+
+// Increments the sent counter for the given peer.
+func (dsl *DefaultScoreLedger) AddToSentBytes(p peer.ID, n int) {
+	l := dsl.findOrCreate(p)
+	l.AddToSentBytes(n)
+}
+
+// Increments the received counter for the given peer.
+func (dsl *DefaultScoreLedger) AddToReceivedBytes(p peer.ID, n int) {
+	l := dsl.findOrCreate(p)
+	l.AddToReceivedBytes(n)
+}
+
+// PeerConnected should be called when a new peer connects, meaning
+// we should open accounting.
+func (dsl *DefaultScoreLedger) PeerConnected(p peer.ID) {
+	dsl.lock.Lock()
+	defer dsl.lock.Unlock()
+	_, ok := dsl.ledgerMap[p]
+	if !ok {
+		dsl.ledgerMap[p] = newScoreLedger(p)
+	}
+}
+
+// PeerDisconnected should be called when a peer disconnects to
+// clean up the accounting.
+func (dsl *DefaultScoreLedger) PeerDisconnected(p peer.ID) {
+	dsl.lock.Lock()
+	defer dsl.lock.Unlock()
+	delete(dsl.ledgerMap, p)
+}
+
+// Creates a new instance of the default score ledger.
+func NewDefaultScoreLedger() *DefaultScoreLedger {
+	return &DefaultScoreLedger{
+		ledgerMap:          make(map[peer.ID]*scoreledger),
+		ticker:             time.NewTicker(time.Millisecond * 100),
+		closing:            make(chan struct{}),
+		peerSampleInterval: shortTerm,
+	}
+}
+
+// Creates a new instance of the default score ledger with testing
+// parameters.
+func NewTestScoreLedger(peerSampleInterval time.Duration, sampleCh chan struct{}) *DefaultScoreLedger {
+	dsl := NewDefaultScoreLedger()
+	dsl.peerSampleInterval = peerSampleInterval
+	dsl.sampleCh = sampleCh
+	return dsl
+}