diff --git a/middleware/hooks.go b/middleware/hooks.go
index 0cfbeac..5269ba4 100644
--- a/middleware/hooks.go
+++ b/middleware/hooks.go
@@ -4,6 +4,7 @@ import (
 	"context"
 
 	"github.com/chihaya/chihaya/bittorrent"
+	"github.com/chihaya/chihaya/storage"
 )
 
 // Hook abstracts the concept of anything that needs to interact with a
@@ -12,3 +13,54 @@ type Hook interface {
 	HandleAnnounce(context.Context, *bittorrent.AnnounceRequest, *bittorrent.AnnounceResponse) (context.Context, error)
 	HandleScrape(context.Context, *bittorrent.ScrapeRequest, *bittorrent.ScrapeResponse) (context.Context, error)
 }
+
+type skipSwarmInteraction struct{}
+
+// SkipSwarmInteractionKey is a key for the context of an Announce to control
+// whether the swarm interaction middleware should run.
+// Any non-nil value set for this key will cause the swarm interaction
+// middleware to skip.
+var SkipSwarmInteractionKey = skipSwarmInteraction{}
+
+type swarmInteractionHook struct {
+	store storage.PeerStore
+}
+
+func (h *swarmInteractionHook) HandleAnnounce(ctx context.Context, req *bittorrent.AnnounceRequest, resp *bittorrent.AnnounceResponse) (_ context.Context, err error) {
+	if ctx.Value(SkipSwarmInteractionKey) != nil {
+		return ctx, nil
+	}
+
+	switch {
+	case req.Event == bittorrent.Stopped:
+		err = h.store.DeleteSeeder(req.InfoHash, req.Peer)
+		if err != nil && err != storage.ErrResourceDoesNotExist {
+			return ctx, err
+		}
+
+		err = h.store.DeleteLeecher(req.InfoHash, req.Peer)
+		if err != nil && err != storage.ErrResourceDoesNotExist {
+			return ctx, err
+		}
+	case req.Event == bittorrent.Completed:
+		err = h.store.GraduateLeecher(req.InfoHash, req.Peer)
+		return ctx, err
+	case req.Left == 0:
+		// Completed events will also have Left == 0, but by making this
+		// an extra case we can treat "old" seeders differently from
+		// graduating leechers. (Calling PutSeeder is probably faster
+		// than calling GraduateLeecher.)
+		err = h.store.PutSeeder(req.InfoHash, req.Peer)
+		return ctx, err
+	default:
+		err = h.store.PutLeecher(req.InfoHash, req.Peer)
+		return ctx, err
+	}
+
+	return ctx, nil
+}
+
+func (h *swarmInteractionHook) HandleScrape(ctx context.Context, _ *bittorrent.ScrapeRequest, _ *bittorrent.ScrapeResponse) (context.Context, error) {
+	// Scrapes have no effect on the swarm.
+	return ctx, nil
+}
diff --git a/middleware/middleware.go b/middleware/middleware.go
index 56cff22..dec1e9f 100644
--- a/middleware/middleware.go
+++ b/middleware/middleware.go
@@ -26,7 +26,7 @@ func NewLogic(cfg Config, peerStore storage.PeerStore, preHooks, postHooks []Hoo
 		announceInterval: cfg.AnnounceInterval,
 		peerStore:        peerStore,
 		preHooks:         preHooks,
-		postHooks:        postHooks,
+		postHooks:        append(postHooks, &swarmInteractionHook{store: peerStore}),
 	}
 
 	return l