diff --git a/api/tag/api.go b/api/tag/context.go
similarity index 58%
rename from api/tag/api.go
rename to api/tag/context.go
index c7250e187b9..b178b6fdb01 100644
--- a/api/tag/api.go
+++ b/api/tag/context.go
@@ -16,8 +16,7 @@ package tag
 
 import (
 	"context"
-
-	"go.opentelemetry.io/api/core"
+	"runtime/pprof"
 )
 
 type ctxTagsType struct{}
@@ -26,56 +25,6 @@ var (
 	ctxTagsKey = &ctxTagsType{}
 )
 
-type MutatorOp int
-
-const (
-	INSERT MutatorOp = iota
-	UPDATE
-	UPSERT
-	DELETE
-)
-
-type Mutator struct {
-	MutatorOp
-	core.KeyValue
-	MeasureMetadata
-}
-
-type MeasureMetadata struct {
-	TTL int // -1 == infinite, 0 == do not propagate
-}
-
-func (m Mutator) WithTTL(hops int) Mutator {
-	m.TTL = hops
-	return m
-}
-
-type MapUpdate struct {
-	SingleKV      core.KeyValue
-	MultiKV       []core.KeyValue
-	SingleMutator Mutator
-	MultiMutator  []Mutator
-}
-
-type Map interface {
-	Apply(MapUpdate) Map
-
-	Value(core.Key) (core.Value, bool)
-	HasValue(core.Key) bool
-
-	Len() int
-
-	Foreach(func(kv core.KeyValue) bool)
-}
-
-func NewEmptyMap() Map {
-	return tagMap{}
-}
-
-func NewMap(update MapUpdate) Map {
-	return NewEmptyMap().Apply(update)
-}
-
 func WithMap(ctx context.Context, m Map) context.Context {
 	return context.WithValue(ctx, ctxTagsKey, m)
 }
@@ -90,5 +39,16 @@ func FromContext(ctx context.Context) Map {
 	if m, ok := ctx.Value(ctxTagsKey).(Map); ok {
 		return m
 	}
-	return tagMap{}
+	return NewEmptyMap()
+}
+
+// Note: the golang pprof.Do API forces this memory allocation, we
+// should file an issue about that.  (There's a TODO in the source.)
+func Do(ctx context.Context, f func(ctx context.Context)) {
+	m := FromContext(ctx)
+	keyvals := make([]string, 0, 2*len(m.m))
+	for k, v := range m.m {
+		keyvals = append(keyvals, k.Variable.Name, v.value.Emit())
+	}
+	pprof.Do(ctx, pprof.Labels(keyvals...), f)
 }
diff --git a/api/tag/map.go b/api/tag/map.go
index 8035470126f..9982627c1b9 100644
--- a/api/tag/map.go
+++ b/api/tag/map.go
@@ -15,64 +15,91 @@
 package tag
 
 import (
-	"context"
-	"runtime/pprof"
-
 	"go.opentelemetry.io/api/core"
 )
 
+type MeasureMetadata struct {
+	TTL int // -1 == infinite, 0 == do not propagate
+}
+
 type tagContent struct {
 	value core.Value
 	meta  MeasureMetadata
 }
 
-type tagMap map[core.Key]tagContent
+type rawMap map[core.Key]tagContent
+
+type Map struct {
+	m rawMap
+}
 
-var _ Map = tagMap{}
+type MapUpdate struct {
+	SingleKV      core.KeyValue
+	MultiKV       []core.KeyValue
+	SingleMutator Mutator
+	MultiMutator  []Mutator
+}
 
-func (t tagMap) Apply(update MapUpdate) Map {
-	m := make(tagMap, len(t)+len(update.MultiKV)+len(update.MultiMutator))
-	for k, v := range t {
-		m[k] = v
+func newMap(raw rawMap) Map {
+	return Map{
+		m: raw,
+	}
+}
+
+func NewEmptyMap() Map {
+	return newMap(nil)
+}
+
+func NewMap(update MapUpdate) Map {
+	return NewEmptyMap().Apply(update)
+}
+
+func (m Map) Apply(update MapUpdate) Map {
+	r := make(rawMap, len(m.m)+len(update.MultiKV)+len(update.MultiMutator))
+	for k, v := range m.m {
+		r[k] = v
 	}
 	if update.SingleKV.Key.Defined() {
-		m[update.SingleKV.Key] = tagContent{
+		r[update.SingleKV.Key] = tagContent{
 			value: update.SingleKV.Value,
 		}
 	}
 	for _, kv := range update.MultiKV {
-		m[kv.Key] = tagContent{
+		r[kv.Key] = tagContent{
 			value: kv.Value,
 		}
 	}
 	if update.SingleMutator.Key.Defined() {
-		m.apply(update.SingleMutator)
+		r.apply(update.SingleMutator)
 	}
 	for _, mutator := range update.MultiMutator {
-		m.apply(mutator)
+		r.apply(mutator)
+	}
+	if len(r) == 0 {
+		r = nil
 	}
-	return m
+	return newMap(r)
 }
 
-func (m tagMap) Value(k core.Key) (core.Value, bool) {
-	entry, ok := m[k]
+func (m Map) Value(k core.Key) (core.Value, bool) {
+	entry, ok := m.m[k]
 	if !ok {
 		entry.value.Type = core.INVALID
 	}
 	return entry.value, ok
 }
 
-func (m tagMap) HasValue(k core.Key) bool {
+func (m Map) HasValue(k core.Key) bool {
 	_, has := m.Value(k)
 	return has
 }
 
-func (m tagMap) Len() int {
-	return len(m)
+func (m Map) Len() int {
+	return len(m.m)
 }
 
-func (m tagMap) Foreach(f func(kv core.KeyValue) bool) {
-	for k, v := range m {
+func (m Map) Foreach(f func(kv core.KeyValue) bool) {
+	for k, v := range m.m {
 		if !f(core.KeyValue{
 			Key:   k,
 			Value: v.value,
@@ -82,10 +109,7 @@ func (m tagMap) Foreach(f func(kv core.KeyValue) bool) {
 	}
 }
 
-func (m tagMap) apply(mutator Mutator) {
-	if m == nil {
-		return
-	}
+func (r rawMap) apply(mutator Mutator) {
 	key := mutator.KeyValue.Key
 	content := tagContent{
 		value: mutator.KeyValue.Value,
@@ -93,57 +117,16 @@ func (m tagMap) apply(mutator Mutator) {
 	}
 	switch mutator.MutatorOp {
 	case INSERT:
-		if _, ok := m[key]; !ok {
-			m[key] = content
+		if _, ok := r[key]; !ok {
+			r[key] = content
 		}
 	case UPDATE:
-		if _, ok := m[key]; ok {
-			m[key] = content
+		if _, ok := r[key]; ok {
+			r[key] = content
 		}
 	case UPSERT:
-		m[key] = content
+		r[key] = content
 	case DELETE:
-		delete(m, key)
-	}
-}
-
-func Insert(kv core.KeyValue) Mutator {
-	return Mutator{
-		MutatorOp: INSERT,
-		KeyValue:  kv,
-	}
-}
-
-func Update(kv core.KeyValue) Mutator {
-	return Mutator{
-		MutatorOp: UPDATE,
-		KeyValue:  kv,
-	}
-}
-
-func Upsert(kv core.KeyValue) Mutator {
-	return Mutator{
-		MutatorOp: UPSERT,
-		KeyValue:  kv,
-	}
-}
-
-func Delete(k core.Key) Mutator {
-	return Mutator{
-		MutatorOp: DELETE,
-		KeyValue: core.KeyValue{
-			Key: k,
-		},
-	}
-}
-
-// Note: the golang pprof.Do API forces this memory allocation, we
-// should file an issue about that.  (There's a TODO in the source.)
-func Do(ctx context.Context, f func(ctx context.Context)) {
-	m := FromContext(ctx).(tagMap)
-	keyvals := make([]string, 0, 2*len(m))
-	for k, v := range m {
-		keyvals = append(keyvals, k.Variable.Name, v.value.Emit())
+		delete(r, key)
 	}
-	pprof.Do(ctx, pprof.Labels(keyvals...), f)
 }
diff --git a/api/tag/mutator.go b/api/tag/mutator.go
new file mode 100644
index 00000000000..13822108803
--- /dev/null
+++ b/api/tag/mutator.go
@@ -0,0 +1,69 @@
+// Copyright 2019, OpenTelemetry Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package tag
+
+import (
+	"go.opentelemetry.io/api/core"
+)
+
+type MutatorOp int
+
+const (
+	INSERT MutatorOp = iota
+	UPDATE
+	UPSERT
+	DELETE
+)
+
+type Mutator struct {
+	MutatorOp
+	core.KeyValue
+	MeasureMetadata
+}
+
+func (m Mutator) WithTTL(hops int) Mutator {
+	m.TTL = hops
+	return m
+}
+
+func Insert(kv core.KeyValue) Mutator {
+	return Mutator{
+		MutatorOp: INSERT,
+		KeyValue:  kv,
+	}
+}
+
+func Update(kv core.KeyValue) Mutator {
+	return Mutator{
+		MutatorOp: UPDATE,
+		KeyValue:  kv,
+	}
+}
+
+func Upsert(kv core.KeyValue) Mutator {
+	return Mutator{
+		MutatorOp: UPSERT,
+		KeyValue:  kv,
+	}
+}
+
+func Delete(k core.Key) Mutator {
+	return Mutator{
+		MutatorOp: DELETE,
+		KeyValue: core.KeyValue{
+			Key: k,
+		},
+	}
+}
diff --git a/experimental/streaming/exporter/reader/format/format.go b/experimental/streaming/exporter/reader/format/format.go
index 45a300aa910..c73ebf5cd0e 100644
--- a/experimental/streaming/exporter/reader/format/format.go
+++ b/experimental/streaming/exporter/reader/format/format.go
@@ -55,7 +55,7 @@ func AppendEvent(buf *strings.Builder, data reader.Event) {
 		} else {
 			buf.WriteString(" <")
 			f(false)(parentSpanIDKey.String(data.Parent.SpanIDString()))
-			if data.ParentAttributes != nil {
+			if data.ParentAttributes.Len() > 0 {
 				data.ParentAttributes.Foreach(f(false))
 			}
 			buf.WriteString(" >")
@@ -113,10 +113,10 @@ func AppendEvent(buf *strings.Builder, data reader.Event) {
 
 	// Attach the scope (span) attributes and context tags.
 	buf.WriteString(" [")
-	if data.Attributes != nil {
+	if data.Attributes.Len() > 0 {
 		data.Attributes.Foreach(f(false))
 	}
-	if data.Tags != nil {
+	if data.Tags.Len() > 0 {
 		data.Tags.Foreach(f(true))
 	}
 	if data.SpanContext.HasSpanID() {
diff --git a/experimental/streaming/exporter/reader/reader.go b/experimental/streaming/exporter/reader/reader.go
index 95204fe998c..c353e9ecaa6 100644
--- a/experimental/streaming/exporter/reader/reader.go
+++ b/experimental/streaming/exporter/reader/reader.go
@@ -311,7 +311,7 @@ func (ro *readerObserver) addMeasurement(e *Event, m stats.Measurement) {
 
 func (ro *readerObserver) readMeasureScope(m stats.Measure) (tag.Map, *readerSpan) {
 	// TODO
-	return nil, nil
+	return tag.NewEmptyMap(), nil
 }
 
 func (ro *readerObserver) readScope(id observer.ScopeID) (tag.Map, *readerSpan) {
diff --git a/sdk/trace/tracer.go b/sdk/trace/tracer.go
index 33aa0012967..a4afd715a48 100644
--- a/sdk/trace/tracer.go
+++ b/sdk/trace/tracer.go
@@ -18,6 +18,7 @@ import (
 	"context"
 
 	"go.opentelemetry.io/api/core"
+	"go.opentelemetry.io/api/tag"
 	apitrace "go.opentelemetry.io/api/trace"
 )
 
@@ -94,5 +95,5 @@ func (tr *tracer) WithComponent(component string) apitrace.Tracer {
 }
 
 func (tr *tracer) Inject(ctx context.Context, span apitrace.Span, injector apitrace.Injector) {
-	injector.Inject(span.SpanContext(), nil)
+	injector.Inject(span.SpanContext(), tag.NewEmptyMap())
 }