diff --git a/pkg/barback/lib/src/asset_cascade.dart b/pkg/barback/lib/src/asset_cascade.dart
index 3454899df30..af0b90c2e48 100644
--- a/pkg/barback/lib/src/asset_cascade.dart
+++ b/pkg/barback/lib/src/asset_cascade.dart
@@ -117,7 +117,7 @@ class AssetCascade {
   /// It loads source assets within [package] using [provider].
   AssetCascade(this.graph, this.package) {
     _onDirtyPool.add(_onDirtyController.stream);
-    _addPhase(new Phase(this, []));
+    _addPhase(new Phase(this, [], package));
 
     // Keep track of logged errors so we can know that the build failed.
     onLog.listen((entry) {
@@ -308,4 +308,6 @@ class AssetCascade {
       return future.then((_) => _process());
     });
   }
+
+  String toString() => "cascade for $package";
 }
diff --git a/pkg/barback/lib/src/asset_node.dart b/pkg/barback/lib/src/asset_node.dart
index a1af547dddd..7ea442abbd5 100644
--- a/pkg/barback/lib/src/asset_node.dart
+++ b/pkg/barback/lib/src/asset_node.dart
@@ -137,6 +137,8 @@ class AssetNode {
       : id = asset.id,
         _asset = asset,
         _state = AssetState.AVAILABLE;
+
+  String toString() => "$state asset $id";
 }
 
 /// The controller for an [AssetNode].
@@ -199,6 +201,8 @@ class AssetNodeController {
     node._asset = asset;
     node._stateChangeController.add(AssetState.AVAILABLE);
   }
+
+  String toString() => "controller for $node";
 }
 
 // TODO(nweiz): add an error state.
diff --git a/pkg/barback/lib/src/asset_set.dart b/pkg/barback/lib/src/asset_set.dart
index 08c2db9b7bf..b17db974914 100644
--- a/pkg/barback/lib/src/asset_set.dart
+++ b/pkg/barback/lib/src/asset_set.dart
@@ -68,4 +68,6 @@ class AssetSet extends IterableBase<Asset> {
   void clear() {
     _assets.clear();
   }
+
+  String toString() => _assets.toString();
 }
diff --git a/pkg/barback/lib/src/group_runner.dart b/pkg/barback/lib/src/group_runner.dart
index 4718d181f70..d2767968bc3 100644
--- a/pkg/barback/lib/src/group_runner.dart
+++ b/pkg/barback/lib/src/group_runner.dart
@@ -13,10 +13,16 @@ import 'phase.dart';
 import 'stream_pool.dart';
 import 'transformer_group.dart';
 
-/// A class that process all of the phases in a single transformer group.
+/// A class that processes all of the phases in a single transformer group.
 ///
 /// A group takes many inputs, processes them, and emits many outputs.
 class GroupRunner {
+  /// The group this runner runs.
+  final TransformerGroup _group;
+
+  /// A string describing the location of [this] in the transformer graph.
+  final String _location;
+
   /// The phases defined by this group.
   final _phases = new List<Phase>();
 
@@ -46,10 +52,10 @@ class GroupRunner {
   /// ensure that it does so.
   final _alreadyEmittedOutputs = new Set<AssetNode>();
 
-  GroupRunner(AssetCascade cascade, TransformerGroup group) {
-    var lastPhase = new Phase(cascade, group.phases.first);
+  GroupRunner(AssetCascade cascade, this._group, this._location) {
+    var lastPhase = new Phase(cascade, _group.phases.first, _location);
     _phases.add(lastPhase);
-    for (var phase in group.phases.skip(1)) {
+    for (var phase in _group.phases.skip(1)) {
       lastPhase = lastPhase.addPhase(phase);
       _phases.add(lastPhase);
     }
@@ -91,4 +97,6 @@ class GroupRunner {
 
     return new Future.value(newOutputs);
   }
+
+  String toString() => "group in phase $_location for $_group";
 }
diff --git a/pkg/barback/lib/src/phase.dart b/pkg/barback/lib/src/phase.dart
index 2136a1281e9..aa570bfd53f 100644
--- a/pkg/barback/lib/src/phase.dart
+++ b/pkg/barback/lib/src/phase.dart
@@ -36,6 +36,12 @@ class Phase {
   /// The cascade that owns this phase.
   final AssetCascade cascade;
 
+  /// A string describing the location of [this] in the transformer graph.
+  final String _location;
+
+  /// The index of [this] in its parent cascade or group.
+  final int _index;
+
   /// The transformers that can access [inputs].
   ///
   /// Their outputs will be available to the next phase.
@@ -111,12 +117,15 @@ class Phase {
   // TODO(nweiz): Rather than passing the cascade and the phase everywhere,
   // create an interface that just exposes [getInput]. Emit errors via
   // [AssetNode]s.
-  Phase(this.cascade, Iterable transformers)
+  Phase(AssetCascade cascade, Iterable transformers, String location)
+      : this._(cascade, transformers, location, 0);
+
+  Phase._(this.cascade, Iterable transformers, this._location, this._index)
       : _transformers = transformers.where((op) => op is Transformer).toSet() {
     _onDirtyPool.add(_onDirtyController.stream);
 
     for (var group in transformers.where((op) => op is TransformerGroup)) {
-      var runner = new GroupRunner(cascade, group);
+      var runner = new GroupRunner(cascade, group, "$_location.$_index");
       _groups[group] = runner;
       _onDirtyPool.add(runner.onDirty);
       _onLogPool.add(runner.onLog);
@@ -148,7 +157,7 @@ class Phase {
     });
 
     _inputOrigins.add(node.origin);
-    var input = new PhaseInput(this, node, _transformers);
+    var input = new PhaseInput(this, node, _transformers, "$_location.$_index");
     _inputs[node.id] = input;
     input.input.whenRemoved(() {
       _inputOrigins.remove(node.origin);
@@ -205,7 +214,7 @@ class Phase {
     }
 
     for (var added in newGroups.difference(oldGroups)) {
-      var runner = new GroupRunner(cascade, added);
+      var runner = new GroupRunner(cascade, added, "$_location.$_index");
       _groups[added] = runner;
       _onDirtyPool.add(runner.onDirty);
       _onLogPool.add(runner.onLog);
@@ -224,7 +233,7 @@ class Phase {
   /// This may only be called on a phase with no phase following it.
   Phase addPhase(Iterable transformers) {
     assert(_next == null);
-    _next = new Phase(cascade, transformers);
+    _next = new Phase._(cascade, transformers, _location, _index + 1);
     for (var output in _outputs.values.toList()) {
       // Remove [output]'s listeners because now they should get the asset from
       // [_next], rather than this phase. Any transforms consuming [output] will
@@ -305,11 +314,13 @@ class Phase {
     if (_outputs.containsKey(asset.id)) {
       _outputs[asset.id].add(asset);
     } else {
-      _outputs[asset.id] = new PhaseOutput(this, asset);
+      _outputs[asset.id] = new PhaseOutput(this, asset, "$_location.$_index");
       _outputs[asset.id].onAsset.listen((output) {
         if (_next != null) _next.addInput(output);
       }, onDone: () => _outputs.remove(asset.id));
       if (_next != null) _next.addInput(_outputs[asset.id].output);
     }
   }
+
+  String toString() => "phase $_location.$_index";
 }
diff --git a/pkg/barback/lib/src/phase_input.dart b/pkg/barback/lib/src/phase_input.dart
index 13bc700c4a6..2168f03b9ee 100644
--- a/pkg/barback/lib/src/phase_input.dart
+++ b/pkg/barback/lib/src/phase_input.dart
@@ -24,6 +24,9 @@ class PhaseInput {
   /// The phase for which this is an input.
   final Phase _phase;
 
+  /// A string describing the location of [this] in the transformer graph.
+  final String _location;
+
   /// The transformers to (potentially) run against [input].
   final Set<Transformer> _transformers;
 
@@ -84,7 +87,8 @@ class PhaseInput {
   Stream<LogEntry> get onLog => _onLogPool.stream;
   final _onLogPool = new StreamPool<LogEntry>.broadcast();
 
-  PhaseInput(this._phase, AssetNode input, Iterable<Transformer> transformers)
+  PhaseInput(this._phase, AssetNode input, Iterable<Transformer> transformers,
+        this._location)
       : _transformers = transformers.toSet(),
         _inputForwarder = new AssetForwarder(input) {
     _onDirtyPool.add(_onDirtyController.stream);
@@ -228,7 +232,8 @@ class PhaseInput {
       // results.
       return transformer.isPrimary(input.asset).then((isPrimary) {
         if (!isPrimary) return;
-        var transform = new TransformNode(_phase, transformer, input);
+        var transform = new TransformNode(
+            _phase, transformer, input, _location);
         _transforms.add(transform);
         _onDirtyPool.add(transform.onDirty);
         _onLogPool.add(transform.onLog);
@@ -312,4 +317,6 @@ class PhaseInput {
       return transform.apply();
     })).then((outputs) => unionAll(outputs));
   }
+
+  String toString() => "phase input in $_location for $input";
 }
diff --git a/pkg/barback/lib/src/phase_output.dart b/pkg/barback/lib/src/phase_output.dart
index 17a613f02ad..50d39c3638d 100644
--- a/pkg/barback/lib/src/phase_output.dart
+++ b/pkg/barback/lib/src/phase_output.dart
@@ -28,6 +28,9 @@ class PhaseOutput {
   /// The phase for which this is an output.
   final Phase _phase;
 
+  /// A string describing the location of [this] in the transformer graph.
+  final String _location;
+
   /// The asset node for this output.
   AssetNode get output => _outputForwarder.node;
   AssetForwarder _outputForwarder;
@@ -53,7 +56,7 @@ class PhaseOutput {
         output.id);
   }
 
-  PhaseOutput(this._phase, AssetNode output)
+  PhaseOutput(this._phase, AssetNode output, this._location)
       : _outputForwarder = new AssetForwarder(output) {
     assert(!output.state.isRemoved);
     add(output);
@@ -109,4 +112,6 @@ class PhaseOutput {
       }
     });
   }
+
+  String toString() => "phase output in $_location for $output";
 }
diff --git a/pkg/barback/lib/src/transform_node.dart b/pkg/barback/lib/src/transform_node.dart
index 76b29bdffd5..8a79b6c6cd5 100644
--- a/pkg/barback/lib/src/transform_node.dart
+++ b/pkg/barback/lib/src/transform_node.dart
@@ -33,6 +33,9 @@ class TransformNode {
   /// The node for the primary asset this transform depends on.
   final AssetNode primary;
 
+  /// A string describing the location of [this] in the transformer graph.
+  final String _location;
+
   /// The subscription to [primary]'s [AssetNode.onStateChange] stream.
   StreamSubscription _primarySubscription;
 
@@ -64,7 +67,7 @@ class TransformNode {
   Stream<LogEntry> get onLog => _onLogController.stream;
   final _onLogController = new StreamController<LogEntry>.broadcast(sync: true);
 
-  TransformNode(this.phase, this.transformer, this.primary) {
+  TransformNode(this.phase, this.transformer, this.primary, this._location) {
     _primarySubscription = primary.onStateChange.listen((state) {
       if (state.isRemoved) {
         remove();
@@ -219,4 +222,7 @@ class TransformNode {
     var entry = new LogEntry(info, asset, level, message, span);
     _onLogController.add(entry);
   }
+
+  String toString() =>
+    "transform node in $_location for $transformer on $primary";
 }
diff --git a/pkg/barback/lib/src/transformer_group.dart b/pkg/barback/lib/src/transformer_group.dart
index 49359c900ce..58ffd2ab39b 100644
--- a/pkg/barback/lib/src/transformer_group.dart
+++ b/pkg/barback/lib/src/transformer_group.dart
@@ -25,4 +25,6 @@ class TransformerGroup {
 
   TransformerGroup(Iterable<Iterable> phases)
       : this.phases = phases.map((phase) => phase.toList()).toList();
+
+  String toString() => "group of $phases";
 }