diff --git a/Makefile b/Makefile
index c313d5dba1..4b4947241d 100644
--- a/Makefile
+++ b/Makefile
@@ -23,7 +23,7 @@ $(SCOPE_EXPORT): $(APP_EXE) $(PROBE_EXE) docker/*
 
 $(APP_EXE): app/*.go report/*.go xfer/*.go
 
-$(PROBE_EXE): probe/*.go report/*.go xfer/*.go
+$(PROBE_EXE): probe/*.go probe/tag/*.go report/*.go xfer/*.go
 
 $(APP_EXE) $(PROBE_EXE):
 	go get -tags netgo ./$(@D)
diff --git a/app/api_topology.go b/app/api_topology.go
index 3fb1d1a38a..0f8af047bf 100644
--- a/app/api_topology.go
+++ b/app/api_topology.go
@@ -30,17 +30,6 @@ type APIEdge struct {
 	Metadata report.AggregateMetadata `json:"metadata"`
 }
 
-// topologySelecter selects a single topology from a report.
-type topologySelecter func(r report.Report) report.Topology
-
-func selectProcess(r report.Report) report.Topology {
-	return r.Process
-}
-
-func selectNetwork(r report.Report) report.Topology {
-	return r.Network
-}
-
 // Full topology.
 func handleTopology(rep Reporter, t topologyView, w http.ResponseWriter, r *http.Request) {
 	respondWith(w, http.StatusOK, APITopology{
diff --git a/app/origin_node.go b/app/origin_node.go
index 0cd96fcf79..fa6b7e432e 100644
--- a/app/origin_node.go
+++ b/app/origin_node.go
@@ -34,8 +34,8 @@ func originNodeForProcess(node report.NodeMetadata) OriginNode {
 		{Key: "Process name", ValueMajor: node["name"], ValueMinor: ""},
 	}
 	for _, tuple := range []struct{ key, human string }{
-		{"docker_id", "Container ID"},
-		{"docker_name", "Container name"},
+		{"docker_container_id", "Container ID"},
+		{"docker_container_name", "Container name"},
 		{"docker_image_id", "Container image ID"},
 		{"docker_image_name", "Container image name"},
 		{"cgroup", "cgroup"},
diff --git a/app/router.go b/app/router.go
index 931f7c2bc9..d6a9dca9a6 100644
--- a/app/router.go
+++ b/app/router.go
@@ -47,16 +47,16 @@ func apiHandler(w http.ResponseWriter, r *http.Request) {
 
 type topologyView struct {
 	human           string
-	selector        topologySelecter
+	selector        report.TopologySelector
 	mapper          report.MapFunc
 	pseudo          report.PseudoFunc
 	groupedTopology string
 }
 
 var topologyRegistry = map[string]topologyView{
-	"applications":         {"Applications", selectProcess, report.ProcessPID, report.GenericPseudoNode, "applications-grouped"},
-	"applications-grouped": {"Applications", selectProcess, report.ProcessName, report.GenericGroupedPseudoNode, ""},
-	"containers":           {"Containers", selectProcess, report.ProcessContainer, report.InternetOnlyPseudoNode, "containers-grouped"},
-	"containers-grouped":   {"Containers", selectProcess, report.ProcessContainerImage, report.InternetOnlyPseudoNode, ""},
-	"hosts":                {"Hosts", selectNetwork, report.NetworkHostname, report.GenericPseudoNode, ""},
+	"applications":         {"Applications", report.SelectProcess, report.ProcessPID, report.GenericPseudoNode, "applications-grouped"},
+	"applications-grouped": {"Applications", report.SelectProcess, report.ProcessName, report.GenericGroupedPseudoNode, ""},
+	"containers":           {"Containers", report.SelectProcess, report.ProcessContainer, report.InternetOnlyPseudoNode, "containers-grouped"},
+	"containers-grouped":   {"Containers", report.SelectProcess, report.ProcessContainerImage, report.InternetOnlyPseudoNode, ""},
+	"hosts":                {"Hosts", report.SelectNetwork, report.NetworkHostname, report.GenericPseudoNode, ""},
 }
diff --git a/probe/docker_process_mapper_test.go b/probe/docker_process_mapper_test.go
deleted file mode 100644
index aaba6f8d61..0000000000
--- a/probe/docker_process_mapper_test.go
+++ /dev/null
@@ -1,100 +0,0 @@
-package main
-
-import (
-	"runtime"
-	"testing"
-	"time"
-
-	docker "github.com/fsouza/go-dockerclient"
-)
-
-type mockDockerClient struct {
-	apiContainers []docker.APIContainers
-	containers    map[string]*docker.Container
-	apiImages     []docker.APIImages
-}
-
-func (m mockDockerClient) ListContainers(options docker.ListContainersOptions) ([]docker.APIContainers, error) {
-	return m.apiContainers, nil
-}
-
-func (m mockDockerClient) InspectContainer(id string) (*docker.Container, error) {
-	return m.containers[id], nil
-}
-
-func (m mockDockerClient) ListImages(options docker.ListImagesOptions) ([]docker.APIImages, error) {
-	return m.apiImages, nil
-}
-
-func (m mockDockerClient) AddEventListener(events chan<- *docker.APIEvents) error {
-	return nil
-}
-
-func (m mockDockerClient) RemoveEventListener(events chan *docker.APIEvents) error {
-	return nil
-}
-
-func TestDockerProcessMapper(t *testing.T) {
-	oldPIDTreeStub, oldDockerClientStub := newPIDTreeStub, newDockerClient
-	defer func() {
-		newPIDTreeStub = oldPIDTreeStub
-		newDockerClient = oldDockerClientStub
-	}()
-
-	newPIDTreeStub = func(procRoot string) (*pidTree, error) {
-		pid1 := &process{pid: 1}
-		pid2 := &process{pid: 2, ppid: 1, parent: pid1}
-		pid1.children = []*process{pid2}
-
-		return &pidTree{
-			processes: map[int]*process{
-				1: pid1, 2: pid2,
-			},
-		}, nil
-	}
-
-	newDockerClient = func(endpoint string) (dockerClient, error) {
-		return mockDockerClient{
-			apiContainers: []docker.APIContainers{{ID: "foo"}},
-			containers: map[string]*docker.Container{
-				"foo": {
-					ID:    "foo",
-					Name:  "bar",
-					Image: "baz",
-					State: docker.State{Pid: 1, Running: true},
-				},
-			},
-			apiImages: []docker.APIImages{{ID: "baz", RepoTags: []string{"tag"}}},
-		}, nil
-	}
-
-	dockerMapper, _ := newDockerMapper("/proc", 10*time.Second)
-	dockerIDMapper := dockerMapper.idMapper()
-	dockerNameMapper := dockerMapper.nameMapper()
-	dockerImageIDMapper := dockerMapper.imageIDMapper()
-	dockerImageNameMapper := dockerMapper.imageNameMapper()
-
-	runtime.Gosched()
-
-	for pid, want := range map[uint]struct{ id, name, imageID, imageName string }{
-		1: {"foo", "bar", "baz", "tag"},
-		2: {"foo", "bar", "baz", "tag"},
-	} {
-		haveID, err := dockerIDMapper.Map(pid)
-		if err != nil || want.id != haveID {
-			t.Errorf("%d: want %q, have %q (%v)", pid, want.id, haveID, err)
-		}
-		haveName, err := dockerNameMapper.Map(pid)
-		if err != nil || want.name != haveName {
-			t.Errorf("%d: want %q, have %q (%v)", pid, want.name, haveName, err)
-		}
-		haveImageID, err := dockerImageIDMapper.Map(pid)
-		if err != nil || want.imageID != haveImageID {
-			t.Errorf("%d: want %q, have %q (%v)", pid, want.imageID, haveImageID, err)
-		}
-		haveImageName, err := dockerImageNameMapper.Map(pid)
-		if err != nil || want.imageName != haveImageName {
-			t.Errorf("%d: want %q, have %q (%v)", pid, want.imageName, haveImageName, err)
-		}
-	}
-}
diff --git a/probe/main.go b/probe/main.go
index de49fd130a..58b8afc3f9 100644
--- a/probe/main.go
+++ b/probe/main.go
@@ -5,7 +5,6 @@ import (
 	"log"
 	"net"
 	"net/http"
-	_ "net/http/pprof"
 	"os"
 	"os/signal"
 	"runtime"
@@ -14,12 +13,12 @@ import (
 	"time"
 
 	"github.com/weaveworks/procspy"
+	"github.com/weaveworks/scope/probe/tag"
 	"github.com/weaveworks/scope/report"
 	"github.com/weaveworks/scope/xfer"
 )
 
-// Set during buildtime.
-var version = "unknown"
+var version = "dev" // set at build time
 
 func main() {
 	var (
@@ -29,9 +28,7 @@ func main() {
 		listen             = flag.String("listen", ":"+strconv.Itoa(xfer.ProbePort), "listen address")
 		prometheusEndpoint = flag.String("prometheus.endpoint", "/metrics", "Prometheus metrics exposition endpoint (requires -http.listen)")
 		spyProcs           = flag.Bool("processes", true, "report processes (needs root)")
-		cgroupsRoot        = flag.String("cgroups.root", "", "if provided, enrich -processes with cgroup names from this root (e.g. /mnt/cgroups)")
-		cgroupsInterval    = flag.Duration("cgroups.interval", 10*time.Second, "how often to update cgroup names")
-		dockerMapper       = flag.Bool("docker", true, "collect Docker-related attributes for processes")
+		dockerTagger       = flag.Bool("docker", true, "collect Docker-related attributes for processes")
 		dockerInterval     = flag.Duration("docker.interval", 10*time.Second, "how often to update Docker attributes")
 		procRoot           = flag.String("proc.root", "/proc", "location of the proc filesystem")
 	)
@@ -42,7 +39,7 @@ func main() {
 		os.Exit(1)
 	}
 
-	log.Printf("probe starting, version %s", version)
+	log.Printf("probe version %s", version)
 
 	procspy.SetProcRoot(*procRoot)
 
@@ -66,32 +63,14 @@ func main() {
 	}
 	defer publisher.Close()
 
-	pms := []processMapper{identityMapper{}}
-
-	if *cgroupsRoot != "" {
-		if fi, err := os.Stat(*cgroupsRoot); err == nil && fi.IsDir() {
-			log.Printf("enriching -processes with cgroup names from %s", *cgroupsRoot)
-			cgroupMapper := newCgroupMapper(*cgroupsRoot, *cgroupsInterval)
-			defer cgroupMapper.Stop()
-			pms = append(pms, cgroupMapper)
-		} else {
-			log.Printf("-cgroups.root=%s: %v", *cgroupsRoot, err)
-		}
-	}
-
-	if *dockerMapper && runtime.GOOS == "Linux" {
-		docker, err := newDockerMapper(*procRoot, *dockerInterval)
+	taggers := []tag.Tagger{tag.NewTopologyTagger()}
+	if *dockerTagger {
+		t, err := tag.NewDockerTagger(*procRoot, *dockerInterval)
 		if err != nil {
-			log.Fatal(err)
+			log.Fatalf("failed to start docker tagger: %v", err)
 		}
-		defer docker.Stop()
-
-		pms = append(pms,
-			docker.idMapper(),
-			docker.nameMapper(),
-			docker.imageIDMapper(),
-			docker.imageNameMapper(),
-		)
+		defer t.Stop()
+		taggers = append(taggers, t)
 	}
 
 	log.Printf("listening on %s", *listen)
@@ -116,7 +95,8 @@ func main() {
 				r = report.MakeReport()
 
 			case <-spyTick:
-				r.Merge(spy(hostname, hostname, *spyProcs, pms))
+				r.Merge(spy(hostname, hostname, *spyProcs))
+				r = tag.Apply(r, taggers)
 				// log.Printf("merged report:\n%#v\n", r)
 
 			case <-quit:
diff --git a/probe/process_mapper.go b/probe/process_mapper.go
deleted file mode 100644
index cfb59307d7..0000000000
--- a/probe/process_mapper.go
+++ /dev/null
@@ -1,143 +0,0 @@
-package main
-
-import (
-	"bufio"
-	"fmt"
-	"log"
-	"os"
-	"path/filepath"
-	"regexp"
-	"strconv"
-	"strings"
-	"sync"
-	"time"
-)
-
-type processMapper interface {
-	Key() string
-	Map(pid uint) (string, error)
-}
-
-type identityMapper struct{}
-
-func (m identityMapper) Key() string                  { return "identity" }
-func (m identityMapper) Map(pid uint) (string, error) { return strconv.FormatUint(uint64(pid), 10), nil }
-
-// cgroupMapper is a cgroup task mapper.
-type cgroupMapper struct {
-	sync.RWMutex
-	root string
-	d    map[uint]string
-	quit chan struct{}
-}
-
-func newCgroupMapper(root string, interval time.Duration) *cgroupMapper {
-	m := cgroupMapper{
-		root: root,
-		d:    map[uint]string{},
-		quit: make(chan struct{}),
-	}
-	m.update()
-	go m.loop(interval)
-	return &m
-}
-
-func (m *cgroupMapper) Stop() {
-	close(m.quit)
-}
-
-func (m *cgroupMapper) Key() string { return "cgroup" }
-
-// Map uses the cache to find the process name for pid. It is safe for
-// concurrent use.
-func (m *cgroupMapper) Map(pid uint) (string, error) {
-	m.RLock()
-	p, ok := m.d[pid]
-	m.RUnlock()
-
-	if !ok {
-		return "", fmt.Errorf("no cgroup for PID %d", pid)
-	}
-
-	return p, nil
-}
-
-func (m *cgroupMapper) loop(d time.Duration) {
-	ticker := time.Tick(d)
-	for {
-		select {
-		case <-ticker:
-			m.update()
-		case <-m.quit:
-			return
-		}
-	}
-}
-
-func (m *cgroupMapper) update() {
-	// We want to read "<root>/<processname>/tasks" files.
-	fh, err := os.Open(m.root)
-	if err != nil {
-		log.Printf("cgroup mapper: %s", err)
-		return
-	}
-
-	dirNames, err := fh.Readdirnames(-1)
-	fh.Close()
-	if err != nil {
-		log.Printf("cgroup mapper: %s", err)
-		return
-	}
-
-	pmap := map[uint]string{}
-	for _, d := range dirNames {
-		cg := normalizeCgroup(d)
-		dirFilename := filepath.Join(m.root, d)
-
-		s, err := os.Stat(dirFilename)
-		if err != nil || !s.IsDir() {
-			continue
-		}
-
-		taskFilename := filepath.Join(dirFilename, "tasks")
-
-		f, err := os.Open(taskFilename)
-		if err != nil {
-			continue
-		}
-
-		r := bufio.NewReader(f)
-		for {
-			line, _, err := r.ReadLine()
-			if err != nil {
-				break // we expect an EOF
-			}
-
-			pid, err := strconv.ParseUint(string(line), 10, 64)
-			if err != nil {
-				log.Printf("continue mapper: %s", err)
-				continue
-			}
-
-			pmap[uint(pid)] = cg
-		}
-
-		f.Close()
-	}
-
-	m.Lock()
-	m.d = pmap
-	m.Unlock()
-}
-
-var lxcRe = regexp.MustCompile(`^([^-]+)-([^-]+)-([A-Fa-f0-9]+)-([0-9]+)$`)
-
-func normalizeCgroup(s string) string {
-	// Format is currently "primarykey-secondarykey-revision-instance". We
-	// want to collapse all instances (and maybe all revisions, in the future)
-	// to the same node. So we remove the instance.
-	if m := lxcRe.FindStringSubmatch(s); len(m) > 0 {
-		return strings.Join([]string{m[1], m[2], m[3]}, "-")
-	}
-	return s
-}
diff --git a/probe/process_mapper_test.go b/probe/process_mapper_test.go
deleted file mode 100644
index 99421a9fc5..0000000000
--- a/probe/process_mapper_test.go
+++ /dev/null
@@ -1,63 +0,0 @@
-package main
-
-import (
-	"io/ioutil"
-	"os"
-	"path"
-	"path/filepath"
-	"testing"
-	"time"
-)
-
-func TestCgroupMapper(t *testing.T) {
-	tmp := setupTmpFS(t, map[string]string{
-		"/systemd/tasks":              "1\n2\n4911\n1000\n25156\n",
-		"/systemd/notify_on_release":  "0\n",
-		"/netscape/tasks":             "666\n4242\n",
-		"/netscape/notify_on_release": "0\n",
-		"/weirdfile":                  "",
-	})
-	defer removeAll(t, tmp)
-
-	m := newCgroupMapper(tmp, 1*time.Second)
-	for pid, want := range map[uint]string{
-		111:   "",
-		999:   "",
-		4911:  "systemd",
-		1:     "systemd", // first one in the file
-		25156: "systemd", // last one in the tasks file
-		4242:  "netscape",
-	} {
-		if have, _ := m.Map(pid); want != have {
-			t.Errorf("%d: want %q, have %q", pid, want, have)
-		}
-	}
-}
-
-func setupTmpFS(t *testing.T, fs map[string]string) string {
-	tmp, err := ioutil.TempDir(os.TempDir(), "scope-probe-test-cgroup-mapper")
-	if err != nil {
-		t.Fatal(err)
-	}
-	//t.Logf("using TempDir %s", tmp)
-
-	for file, content := range fs {
-		dir := path.Dir(file)
-		if err := os.MkdirAll(filepath.Join(tmp, dir), 0777); err != nil {
-			removeAll(t, tmp)
-			t.Fatalf("MkdirAll: %v", err)
-		}
-
-		if err := ioutil.WriteFile(filepath.Join(tmp, file), []byte(content), 0655); err != nil {
-			removeAll(t, tmp)
-			t.Fatalf("WriteFile: %v", err)
-		}
-	}
-	return tmp
-}
-
-func removeAll(t *testing.T, path string) {
-	if err := os.RemoveAll(path); err != nil {
-		t.Error(err)
-	}
-}
diff --git a/probe/spy.go b/probe/spy.go
index 5d72464210..5d40a2bd48 100644
--- a/probe/spy.go
+++ b/probe/spy.go
@@ -18,7 +18,6 @@ import (
 func spy(
 	hostID, hostName string,
 	includeProcesses bool,
-	pms []processMapper,
 ) report.Report {
 	defer func(begin time.Time) {
 		spyDuration.WithLabelValues().Observe(float64(time.Since(begin)))
@@ -33,7 +32,7 @@ func spy(
 	}
 
 	for conn := conns.Next(); conn != nil; conn = conns.Next() {
-		addConnection(&r, conn, hostID, hostName, pms)
+		addConnection(&r, conn, hostID, hostName)
 	}
 
 	return r
@@ -43,7 +42,6 @@ func addConnection(
 	r *report.Report,
 	c *procspy.Connection,
 	hostID, hostName string,
-	pms []processMapper,
 ) {
 	var (
 		scopedLocal  = scopedIP(hostID, c.LocalAddress)
@@ -84,14 +82,6 @@ func addConnection(
 				"domain": hostID,
 			}
 
-			for _, pm := range pms {
-				v, err := pm.Map(c.PID)
-				if err != nil {
-					continue
-				}
-				md[pm.Key()] = v
-			}
-
 			r.Process.NodeMetadatas[scopedLocal] = md
 		}
 		// Count the TCP connection.
diff --git a/probe/spy_test.go b/probe/spy_test.go
index a7e80df1c1..04f5319e86 100644
--- a/probe/spy_test.go
+++ b/probe/spy_test.go
@@ -87,7 +87,7 @@ func TestSpyNetwork(t *testing.T) {
 		nodeName = "frenchs-since-1904"
 	)
 
-	r := spy(nodeID, nodeName, false, []processMapper{})
+	r := spy(nodeID, nodeName, false)
 	//buf, _ := json.MarshalIndent(r, "", "    ")
 	//t.Logf("\n%s\n", buf)
 
@@ -123,7 +123,7 @@ func TestSpyProcess(t *testing.T) {
 		nodeName = "fishermans-friend"
 	)
 
-	r := spy(nodeID, nodeName, true, []processMapper{})
+	r := spy(nodeID, nodeName, true)
 	// buf, _ := json.MarshalIndent(r, "", "    ") ; t.Logf("\n%s\n", buf)
 
 	var (
@@ -150,26 +150,3 @@ func TestSpyProcess(t *testing.T) {
 		}
 	}
 }
-
-func TestSpyProcessDataSource(t *testing.T) {
-	procspy.SetFixtures(fixConnectionsWithProcesses)
-
-	const (
-		nodeID   = "chianti"
-		nodeName = "harmonisch"
-	)
-
-	m := identityMapper{}
-	r := spy(nodeID, nodeName, true, []processMapper{m})
-	scopedLocal := scopedIPPort(nodeID, fixLocalAddress, fixLocalPort)
-
-	k := m.Key()
-	v, err := m.Map(fixProcessPID)
-	if err != nil {
-		t.Fatal(err)
-	}
-
-	if want, have := v, r.Process.NodeMetadatas[scopedLocal][k]; want != have {
-		t.Fatalf("%s: want %q, have %q", k, want, have)
-	}
-}
diff --git a/probe/docker_process_mapper.go b/probe/tag/docker_tagger.go
similarity index 50%
rename from probe/docker_process_mapper.go
rename to probe/tag/docker_tagger.go
index 6687932323..e4b9b34e97 100644
--- a/probe/docker_process_mapper.go
+++ b/probe/tag/docker_tagger.go
@@ -1,13 +1,14 @@
-package main
+package tag
 
 import (
-	"fmt"
 	"log"
+	"strconv"
 	"strings"
 	"sync"
 	"time"
 
 	docker "github.com/fsouza/go-dockerclient"
+	"github.com/weaveworks/scope/report"
 )
 
 const (
@@ -15,7 +16,14 @@ const (
 	start = "start"
 )
 
-type dockerMapper struct {
+var (
+	newDockerClientStub = newDockerClient
+	newPIDTreeStub      = newPIDTree
+)
+
+// DockerTagger is a tagger that tags Docker container information to process
+// nodes that have a PID.
+type DockerTagger struct {
 	sync.RWMutex
 	quit     chan struct{}
 	interval time.Duration
@@ -28,13 +36,14 @@ type dockerMapper struct {
 	pidTree  *pidTree
 }
 
-func newDockerMapper(procRoot string, interval time.Duration) (*dockerMapper, error) {
+// NewDockerTagger returns a usable DockerTagger. Don't forget to Stop it.
+func NewDockerTagger(procRoot string, interval time.Duration) (*DockerTagger, error) {
 	pidTree, err := newPIDTreeStub(procRoot)
 	if err != nil {
 		return nil, err
 	}
 
-	m := dockerMapper{
+	t := DockerTagger{
 		containers:      map[string]*docker.Container{},
 		containersByPID: map[int]*docker.Container{},
 		images:          map[string]*docker.APIImages{},
@@ -46,34 +55,35 @@ func newDockerMapper(procRoot string, interval time.Duration) (*dockerMapper, er
 		quit:     make(chan struct{}),
 	}
 
-	go m.loop()
-	return &m, nil
+	go t.loop()
+	return &t, nil
 }
 
-func (m *dockerMapper) Stop() {
-	close(m.quit)
+// Stop stops the Docker tagger's event subscriber.
+func (t *DockerTagger) Stop() {
+	close(t.quit)
 }
 
-func (m *dockerMapper) loop() {
-	if !m.update() {
+func (t *DockerTagger) loop() {
+	if !t.update() {
 		return
 	}
 
-	ticker := time.Tick(m.interval)
+	ticker := time.Tick(t.interval)
 	for {
 		select {
 		case <-ticker:
-			if !m.update() {
+			if !t.update() {
 				return
 			}
 
-		case <-m.quit:
+		case <-t.quit:
 			return
 		}
 	}
 }
 
-// for mocking
+// Sub-interface for mocking.
 type dockerClient interface {
 	ListContainers(docker.ListContainersOptions) ([]docker.APIContainers, error)
 	InspectContainer(string) (*docker.Container, error)
@@ -82,19 +92,13 @@ type dockerClient interface {
 	RemoveEventListener(chan *docker.APIEvents) error
 }
 
-func newRealDockerClient(endpoint string) (dockerClient, error) {
+func newDockerClient(endpoint string) (dockerClient, error) {
 	return docker.NewClient(endpoint)
 }
 
-var (
-	newDockerClient = newRealDockerClient
-	newPIDTreeStub  = newPIDTree
-)
-
-// returns false when stopping.
-func (m *dockerMapper) update() bool {
+func (t *DockerTagger) update() bool {
 	endpoint := "unix:///var/run/docker.sock"
-	client, err := newDockerClient(endpoint)
+	client, err := newDockerClientStub(endpoint)
 	if err != nil {
 		log.Printf("docker mapper: %s", err)
 		return true
@@ -111,40 +115,40 @@ func (m *dockerMapper) update() bool {
 		}
 	}()
 
-	if err := m.updateContainers(client); err != nil {
+	if err := t.updateContainers(client); err != nil {
 		log.Printf("docker mapper: %s", err)
 		return true
 	}
 
-	if err := m.updateImages(client); err != nil {
+	if err := t.updateImages(client); err != nil {
 		log.Printf("docker mapper: %s", err)
 		return true
 	}
 
-	otherUpdates := time.Tick(m.interval)
+	otherUpdates := time.Tick(t.interval)
 	for {
 		select {
 		case event := <-events:
-			m.handleEvent(event, client)
+			t.handleEvent(event, client)
 
 		case <-otherUpdates:
-			if err := m.updatePIDTree(); err != nil {
+			if err := t.updatePIDTree(); err != nil {
 				log.Printf("docker mapper: %s", err)
 				continue
 			}
 
-			if err := m.updateImages(client); err != nil {
+			if err := t.updateImages(client); err != nil {
 				log.Printf("docker mapper: %s", err)
 				continue
 			}
 
-		case <-m.quit:
+		case <-t.quit:
 			return false
 		}
 	}
 }
 
-func (m *dockerMapper) updateContainers(client dockerClient) error {
+func (t *DockerTagger) updateContainers(client dockerClient) error {
 	apiContainers, err := client.ListContainers(docker.ListContainersOptions{All: true})
 	if err != nil {
 		return err
@@ -165,44 +169,44 @@ func (m *dockerMapper) updateContainers(client dockerClient) error {
 		containers = append(containers, container)
 	}
 
-	m.Lock()
+	t.Lock()
 	for _, container := range containers {
-		m.containers[container.ID] = container
-		m.containersByPID[container.State.Pid] = container
+		t.containers[container.ID] = container
+		t.containersByPID[container.State.Pid] = container
 	}
-	m.Unlock()
+	t.Unlock()
 
 	return nil
 }
 
-func (m *dockerMapper) updateImages(client dockerClient) error {
+func (t *DockerTagger) updateImages(client dockerClient) error {
 	images, err := client.ListImages(docker.ListImagesOptions{})
 	if err != nil {
 		return err
 	}
 
-	m.Lock()
+	t.Lock()
 	for i := range images {
 		image := &images[i]
-		m.images[image.ID] = image
+		t.images[image.ID] = image
 	}
-	m.Unlock()
+	t.Unlock()
 
 	return nil
 }
 
-func (m *dockerMapper) handleEvent(event *docker.APIEvents, client dockerClient) {
+func (t *DockerTagger) handleEvent(event *docker.APIEvents, client dockerClient) {
 	switch event.Status {
 	case stop:
 		containerID := event.ID
-		m.Lock()
-		if container, ok := m.containers[containerID]; ok {
-			delete(m.containers, containerID)
-			delete(m.containersByPID, container.State.Pid)
+		t.Lock()
+		if container, ok := t.containers[containerID]; ok {
+			delete(t.containers, containerID)
+			delete(t.containersByPID, container.State.Pid)
 		} else {
 			log.Printf("docker mapper: container %s not found", containerID)
 		}
-		m.Unlock()
+		t.Unlock()
 
 	case start:
 		containerID := event.ID
@@ -217,88 +221,77 @@ func (m *dockerMapper) handleEvent(event *docker.APIEvents, client dockerClient)
 			return
 		}
 
-		m.Lock()
-		m.containers[containerID] = container
-		m.containersByPID[container.State.Pid] = container
-		m.Unlock()
+		t.Lock()
+		t.containers[containerID] = container
+		t.containersByPID[container.State.Pid] = container
+		t.Unlock()
 	}
 }
 
-func (m *dockerMapper) updatePIDTree() error {
-	pidTree, err := newPIDTreeStub(m.procRoot)
+func (t *DockerTagger) updatePIDTree() error {
+	pidTree, err := newPIDTreeStub(t.procRoot)
 	if err != nil {
 		return err
 	}
 
-	m.Lock()
-	m.pidTree = pidTree
-	m.Unlock()
+	t.Lock()
+	t.pidTree = pidTree
+	t.Unlock()
 	return nil
 }
 
-type dockerProcessMapper struct {
-	*dockerMapper
-	key string
-	f   func(*docker.Container) string
-}
-
-func (m *dockerProcessMapper) Key() string { return m.key }
-func (m *dockerProcessMapper) Map(pid uint) (string, error) {
-	var (
-		container *docker.Container
-		ok        bool
-		err       error
-		candidate = int(pid)
-	)
-
-	m.RLock()
-	for {
-		container, ok = m.containersByPID[candidate]
-		if ok {
-			break
+// Tag implements Tagger.
+func (t *DockerTagger) Tag(r report.Report) report.Report {
+	for nodeID, nodeMetadata := range r.Process.NodeMetadatas {
+		pidStr, ok := nodeMetadata["pid"]
+		if !ok {
+			//log.Printf("dockerTagger: %q: no process node ID", id)
+			continue
 		}
-		candidate, err = m.pidTree.getParent(candidate)
+		pid, err := strconv.ParseUint(pidStr, 10, 64)
 		if err != nil {
-			break
+			//log.Printf("dockerTagger: %q: bad process node PID (%v)", id, err)
+			continue
 		}
-	}
-	m.RUnlock()
-
-	if err != nil {
-		return "", fmt.Errorf("no container found for PID %d", pid)
-	}
 
-	return m.f(container), nil
-}
+		var (
+			container *docker.Container
+			candidate = int(pid)
+		)
 
-func (m *dockerMapper) idMapper() processMapper {
-	return &dockerProcessMapper{m, "docker_id", func(c *docker.Container) string {
-		return c.ID
-	}}
-}
+		t.RLock()
+		for {
+			container, ok = t.containersByPID[candidate]
+			if ok {
+				break
+			}
+			candidate, err = t.pidTree.getParent(candidate)
+			if err != nil {
+				break
+			}
+		}
+		t.RUnlock()
 
-func (m *dockerMapper) nameMapper() processMapper {
-	return &dockerProcessMapper{m, "docker_name", func(c *docker.Container) string {
-		return strings.TrimPrefix(c.Name, "/")
-	}}
-}
+		if !ok {
+			continue
+		}
 
-func (m *dockerMapper) imageIDMapper() processMapper {
-	return &dockerProcessMapper{m, "docker_image_id", func(c *docker.Container) string {
-		return c.Image
-	}}
-}
+		md := report.NodeMetadata{
+			"docker_container_id":   container.ID,
+			"docker_container_name": strings.TrimPrefix(container.Name, "/"),
+			"docker_image_id":       container.Image,
+		}
 
-func (m *dockerMapper) imageNameMapper() processMapper {
-	return &dockerProcessMapper{m, "docker_image_name", func(c *docker.Container) string {
-		m.RLock()
-		image, ok := m.images[c.Image]
-		m.RUnlock()
+		t.RLock()
+		image, ok := t.images[container.Image]
+		t.RUnlock()
 
-		if !ok || len(image.RepoTags) == 0 {
-			return ""
+		if ok && len(image.RepoTags) > 0 {
+			md["docker_image_name"] = image.RepoTags[0]
 		}
 
-		return image.RepoTags[0]
-	}}
+		r.Process.NodeMetadatas[nodeID].Merge(md)
+	}
+
+	return r
 }
diff --git a/probe/tag/docker_tagger_test.go b/probe/tag/docker_tagger_test.go
new file mode 100644
index 0000000000..682e78a62d
--- /dev/null
+++ b/probe/tag/docker_tagger_test.go
@@ -0,0 +1,94 @@
+package tag
+
+import (
+	"reflect"
+	"runtime"
+	"testing"
+	"time"
+
+	docker "github.com/fsouza/go-dockerclient"
+	"github.com/weaveworks/scope/report"
+)
+
+type mockDockerClient struct {
+	apiContainers []docker.APIContainers
+	containers    map[string]*docker.Container
+	apiImages     []docker.APIImages
+}
+
+func (m mockDockerClient) ListContainers(docker.ListContainersOptions) ([]docker.APIContainers, error) {
+	return m.apiContainers, nil
+}
+
+func (m mockDockerClient) InspectContainer(id string) (*docker.Container, error) {
+	return m.containers[id], nil
+}
+
+func (m mockDockerClient) ListImages(docker.ListImagesOptions) ([]docker.APIImages, error) {
+	return m.apiImages, nil
+}
+
+func (m mockDockerClient) AddEventListener(events chan<- *docker.APIEvents) error {
+	return nil
+}
+
+func (m mockDockerClient) RemoveEventListener(events chan *docker.APIEvents) error {
+	return nil
+}
+
+func TestDockerTagger(t *testing.T) {
+	oldPIDTree, oldDockerClient := newPIDTreeStub, newDockerClientStub
+	defer func() { newPIDTreeStub, newDockerClientStub = oldPIDTree, oldDockerClient }()
+
+	newPIDTreeStub = func(procRoot string) (*pidTree, error) {
+		pid1 := &process{pid: 1}
+		pid2 := &process{pid: 2, ppid: 1, parent: pid1}
+		pid1.children = []*process{pid2}
+		return &pidTree{
+			processes: map[int]*process{
+				1: pid1, 2: pid2,
+			},
+		}, nil
+	}
+
+	newDockerClientStub = func(endpoint string) (dockerClient, error) {
+		return mockDockerClient{
+			apiContainers: []docker.APIContainers{{ID: "foo"}},
+			containers: map[string]*docker.Container{
+				"foo": {
+					ID:    "foo",
+					Name:  "bar",
+					Image: "baz",
+					State: docker.State{Pid: 1, Running: true},
+				},
+			},
+			apiImages: []docker.APIImages{{ID: "baz", RepoTags: []string{"bang", "not-chosen"}}},
+		}, nil
+	}
+
+	var (
+		endpoint1NodeID     = "somehost.com;192.168.1.1;12345"
+		endpoint2NodeID     = "somehost.com;192.168.1.1;67890"
+		processNodeMetadata = report.NodeMetadata{
+			"docker_container_id":   "foo",
+			"docker_container_name": "bar",
+			"docker_image_id":       "baz",
+			"docker_image_name":     "bang",
+		}
+	)
+
+	r := report.MakeReport()
+	r.Process.NodeMetadatas[endpoint1NodeID] = report.NodeMetadata{"pid": "1"}
+	r.Process.NodeMetadatas[endpoint2NodeID] = report.NodeMetadata{"pid": "2"}
+
+	dockerTagger, _ := NewDockerTagger("/irrelevant", 10*time.Second)
+	runtime.Gosched()
+	for _, endpointNodeID := range []string{endpoint1NodeID, endpoint2NodeID} {
+		want := processNodeMetadata.Copy()
+		have := dockerTagger.Tag(r).Process.NodeMetadatas[endpointNodeID].Copy()
+		delete(have, "pid")
+		if !reflect.DeepEqual(want, have) {
+			t.Errorf("%q: want %+v, have %+v", endpointNodeID, want, have)
+		}
+	}
+}
diff --git a/probe/pidtree.go b/probe/tag/pidtree.go
similarity index 99%
rename from probe/pidtree.go
rename to probe/tag/pidtree.go
index 65d0f2d61e..fe665be9ae 100644
--- a/probe/pidtree.go
+++ b/probe/tag/pidtree.go
@@ -1,4 +1,4 @@
-package main
+package tag
 
 import (
 	"fmt"
diff --git a/probe/pidtree_test.go b/probe/tag/pidtree_test.go
similarity index 96%
rename from probe/pidtree_test.go
rename to probe/tag/pidtree_test.go
index f83fbd4971..fe4805f07f 100644
--- a/probe/pidtree_test.go
+++ b/probe/tag/pidtree_test.go
@@ -1,4 +1,4 @@
-package main
+package tag
 
 import (
 	"fmt"
@@ -48,7 +48,7 @@ func TestPIDTree(t *testing.T) {
 		return []byte(fmt.Sprintf("%d na R %d", pid, parent)), nil
 	}
 
-	pidtree, err := newPIDTree("/proc")
+	pidtree, err := newPIDTreeStub("/proc")
 	if err != nil {
 		t.Fatalf("newPIDTree error: %v", err)
 	}
diff --git a/probe/tag/tagger.go b/probe/tag/tagger.go
new file mode 100644
index 0000000000..fb1afa4edb
--- /dev/null
+++ b/probe/tag/tagger.go
@@ -0,0 +1,16 @@
+package tag
+
+import "github.com/weaveworks/scope/report"
+
+// Tagger tags nodes with value-add node metadata.
+type Tagger interface {
+	Tag(r report.Report) report.Report
+}
+
+// Apply tags the report with all the taggers.
+func Apply(r report.Report, taggers []Tagger) report.Report {
+	for _, tagger := range taggers {
+		r = tagger.Tag(r)
+	}
+	return r
+}
diff --git a/probe/tag/tagger_test.go b/probe/tag/tagger_test.go
new file mode 100644
index 0000000000..6f6caa906d
--- /dev/null
+++ b/probe/tag/tagger_test.go
@@ -0,0 +1,44 @@
+package tag_test
+
+import (
+	"reflect"
+	"testing"
+
+	"github.com/weaveworks/scope/probe/tag"
+	"github.com/weaveworks/scope/report"
+)
+
+func TestApply(t *testing.T) {
+	var (
+		processNodeID       = "c"
+		networkNodeID       = "d"
+		processNodeMetadata = report.NodeMetadata{"5": "6"}
+		networkNodeMetadata = report.NodeMetadata{"7": "8"}
+	)
+
+	r := report.MakeReport()
+	r.Process.NodeMetadatas[processNodeID] = processNodeMetadata
+	r.Network.NodeMetadatas[networkNodeID] = networkNodeMetadata
+	r = tag.Apply(r, []tag.Tagger{tag.NewTopologyTagger()})
+
+	for _, tuple := range []struct {
+		want report.NodeMetadata
+		from report.Topology
+		via  string
+	}{
+		{copy(processNodeMetadata).Merge(report.NodeMetadata{"topology": "process"}), r.Process, processNodeID},
+		{copy(networkNodeMetadata).Merge(report.NodeMetadata{"topology": "network"}), r.Network, networkNodeID},
+	} {
+		if want, have := tuple.want, tuple.from.NodeMetadatas[tuple.via]; !reflect.DeepEqual(want, have) {
+			t.Errorf("want %+v, have %+v", want, have)
+		}
+	}
+}
+
+func copy(input report.NodeMetadata) report.NodeMetadata {
+	output := make(report.NodeMetadata, len(input))
+	for k, v := range input {
+		output[k] = v
+	}
+	return output
+}
diff --git a/probe/tag/topology_tagger.go b/probe/tag/topology_tagger.go
new file mode 100644
index 0000000000..309fd52837
--- /dev/null
+++ b/probe/tag/topology_tagger.go
@@ -0,0 +1,25 @@
+package tag
+
+import (
+	"github.com/weaveworks/scope/report"
+)
+
+type topologyTagger struct{}
+
+// NewTopologyTagger tags each node with the topology that it comes from.
+func NewTopologyTagger() Tagger {
+	return &topologyTagger{}
+}
+
+func (topologyTagger) Tag(r report.Report) report.Report {
+	for val, topology := range map[string]*report.Topology{
+		"process": &(r.Process),
+		"network": &(r.Network),
+	} {
+		md := report.NodeMetadata{"topology": val}
+		for nodeID := range topology.NodeMetadatas {
+			(*topology).NodeMetadatas[nodeID].Merge(md)
+		}
+	}
+	return r
+}
diff --git a/probe/tag/topology_tagger_test.go b/probe/tag/topology_tagger_test.go
new file mode 100644
index 0000000000..00c628ace5
--- /dev/null
+++ b/probe/tag/topology_tagger_test.go
@@ -0,0 +1,19 @@
+package tag_test
+
+import (
+	"reflect"
+	"testing"
+
+	"github.com/weaveworks/scope/probe/tag"
+	"github.com/weaveworks/scope/report"
+)
+
+func TestTagMissingID(t *testing.T) {
+	const nodeID = "not-found"
+	r := report.MakeReport()
+	want := report.NodeMetadata{}
+	have := tag.NewTopologyTagger().Tag(r).Process.NodeMetadatas[nodeID].Copy()
+	if !reflect.DeepEqual(want, have) {
+		t.Error("TopologyTagger erroneously tagged a missing node ID")
+	}
+}
diff --git a/report/mapping_functions.go b/report/mapping_functions.go
index f739edcba3..e04024ebc9 100644
--- a/report/mapping_functions.go
+++ b/report/mapping_functions.go
@@ -33,6 +33,19 @@ type MapFunc func(string, NodeMetadata) (MappedNode, bool)
 // node IDs prior to mapping.
 type PseudoFunc func(srcNodeID string, srcNode RenderableNode, dstNodeID string) (MappedNode, bool)
 
+// TopologySelector selects a single topology from a report.
+type TopologySelector func(r Report) Topology
+
+// SelectProcess selects the process topology.
+func SelectProcess(r Report) Topology {
+	return r.Process
+}
+
+// SelectNetwork selects the network topology.
+func SelectNetwork(r Report) Topology {
+	return r.Network
+}
+
 // ProcessPID takes a node NodeMetadata from a Process topology, and returns a
 // representation with the ID based on the process PID and the labels based
 // on the process name.
@@ -70,10 +83,10 @@ func ProcessName(_ string, m NodeMetadata) (MappedNode, bool) {
 // are grouped into the Uncontained node.
 func ProcessContainer(_ string, m NodeMetadata) (MappedNode, bool) {
 	var id, major, minor, rank string
-	if m["docker_id"] == "" {
+	if m["docker_container_id"] == "" {
 		id, major, minor, rank = "uncontained", "Uncontained", "", "uncontained"
 	} else {
-		id, major, minor, rank = m["docker_id"], m["docker_name"], m["domain"], m["docker_image_id"]
+		id, major, minor, rank = m["docker_container_id"], m["docker_container_name"], m["domain"], m["docker_image_id"]
 	}
 
 	return MappedNode{
diff --git a/report/mapping_test.go b/report/mapping_test.go
index 79a1eabbee..c413fa62fa 100644
--- a/report/mapping_test.go
+++ b/report/mapping_test.go
@@ -69,13 +69,13 @@ func TestUngroupedMapping(t *testing.T) {
 			f:  ProcessContainer,
 			id: "bar-id",
 			meta: NodeMetadata{
-				"pid":               "42",
-				"name":              "curl",
-				"domain":            "hosta",
-				"docker_id":         "d321fe0",
-				"docker_name":       "walking_sparrow",
-				"docker_image_id":   "1101fff",
-				"docker_image_name": "org/app:latest",
+				"pid":                   "42",
+				"name":                  "curl",
+				"domain":                "hosta",
+				"docker_container_id":   "d321fe0",
+				"docker_container_name": "walking_sparrow",
+				"docker_image_id":       "1101fff",
+				"docker_image_name":     "org/app:latest",
 			},
 			wantOK:    true,
 			wantID:    "d321fe0",
diff --git a/report/topology.go b/report/topology.go
index d9b4541551..0836e02928 100644
--- a/report/topology.go
+++ b/report/topology.go
@@ -54,6 +54,26 @@ type EdgeMetadata struct {
 // which should probably change (see comment on type MapFunc).
 type NodeMetadata map[string]string
 
+// Copy returns a value copy, useful for tests.
+func (nm NodeMetadata) Copy() NodeMetadata {
+	cp := make(NodeMetadata, len(nm))
+	for k, v := range nm {
+		cp[k] = v
+	}
+	return cp
+}
+
+// Merge merges two node metadata maps together. In case of conflict, the
+// other (right-hand) side wins. Always reassign the result of merge to the
+// destination. Merge is defined on the value-type, but node metadata map is
+// itself a reference type, so if you want to maintain immutability, use copy.
+func (nm NodeMetadata) Merge(other NodeMetadata) NodeMetadata {
+	for k, v := range other {
+		nm[k] = v // other takes precedence
+	}
+	return nm
+}
+
 // NewTopology gives you a Topology.
 func NewTopology() Topology {
 	return Topology{