diff --git a/pkg/apis/autoscaling/validation/validation.go b/pkg/apis/autoscaling/validation/validation.go
index 6338b2a744e..a8e789cf8a3 100644
--- a/pkg/apis/autoscaling/validation/validation.go
+++ b/pkg/apis/autoscaling/validation/validation.go
@@ -115,7 +115,7 @@ func validateMetrics(metrics []autoscaling.MetricSpec, fldPath *field.Path) fiel
 	return allErrs
 }
 
-var validMetricSourceTypes = sets.NewString(string(autoscaling.ObjectMetricSourceType), string(autoscaling.PodsMetricSourceType), string(autoscaling.ResourceMetricSourceType))
+var validMetricSourceTypes = sets.NewString(string(autoscaling.ObjectMetricSourceType), string(autoscaling.PodsMetricSourceType), string(autoscaling.ResourceMetricSourceType), string(autoscaling.ExternalMetricSourceType))
 var validMetricSourceTypesList = validMetricSourceTypes.List()
 
 func validateMetricSpec(spec autoscaling.MetricSpec, fldPath *field.Path) field.ErrorList {
@@ -137,6 +137,13 @@ func validateMetricSpec(spec autoscaling.MetricSpec, fldPath *field.Path) field.
 		}
 	}
 
+	if spec.External != nil {
+		typesPresent.Insert("external")
+		if typesPresent.Len() == 1 {
+			allErrs = append(allErrs, validateExternalSource(spec.External, fldPath.Child("external"))...)
+		}
+	}
+
 	if spec.Pods != nil {
 		typesPresent.Insert("pods")
 		if typesPresent.Len() == 1 {
@@ -183,6 +190,32 @@ func validateObjectSource(src *autoscaling.ObjectMetricSource, fldPath *field.Pa
 	return allErrs
 }
 
+func validateExternalSource(src *autoscaling.ExternalMetricSource, fldPath *field.Path) field.ErrorList {
+	allErrs := field.ErrorList{}
+
+	if len(src.MetricName) == 0 {
+		allErrs = append(allErrs, field.Required(fldPath.Child("metricName"), "must specify a metric name"))
+	}
+
+	if src.TargetValue == nil && src.TargetAverageValue == nil {
+		allErrs = append(allErrs, field.Required(fldPath.Child("targetValue"), "must set either a target value for metric or a per-pod target"))
+	}
+
+	if src.TargetValue != nil && src.TargetAverageValue != nil {
+		allErrs = append(allErrs, field.Forbidden(fldPath.Child("targetValue"), "may not set both a target value for metric and a per-pod target"))
+	}
+
+	if src.TargetAverageValue != nil && src.TargetAverageValue.Sign() != 1 {
+		allErrs = append(allErrs, field.Invalid(fldPath.Child("targetAverageValue"), src.TargetAverageValue, "must be positive"))
+	}
+
+	if src.TargetValue != nil && src.TargetValue.Sign() != 1 {
+		allErrs = append(allErrs, field.Invalid(fldPath.Child("targetValue"), src.TargetValue, "must be positive"))
+	}
+
+	return allErrs
+}
+
 func validatePodsSource(src *autoscaling.PodsMetricSource, fldPath *field.Path) field.ErrorList {
 	allErrs := field.ErrorList{}
 
diff --git a/pkg/apis/autoscaling/validation/validation_test.go b/pkg/apis/autoscaling/validation/validation_test.go
index d334a77b3c9..665afd50209 100644
--- a/pkg/apis/autoscaling/validation/validation_test.go
+++ b/pkg/apis/autoscaling/validation/validation_test.go
@@ -202,6 +202,62 @@ func TestValidateHorizontalPodAutoscaler(t *testing.T) {
 				},
 			},
 		},
+		{
+			ObjectMeta: metav1.ObjectMeta{
+				Name:      "myautoscaler",
+				Namespace: metav1.NamespaceDefault,
+			},
+			Spec: autoscaling.HorizontalPodAutoscalerSpec{
+				ScaleTargetRef: autoscaling.CrossVersionObjectReference{
+					Kind: "ReplicationController",
+					Name: "myrc",
+				},
+				MinReplicas: newInt32(1),
+				MaxReplicas: 5,
+				Metrics: []autoscaling.MetricSpec{
+					{
+						Type: autoscaling.ExternalMetricSourceType,
+						External: &autoscaling.ExternalMetricSource{
+							MetricName: "some/metric",
+							MetricSelector: &metav1.LabelSelector{
+								MatchLabels: map[string]string{
+									"label": "value",
+								},
+							},
+							TargetValue: resource.NewMilliQuantity(300, resource.DecimalSI),
+						},
+					},
+				},
+			},
+		},
+		{
+			ObjectMeta: metav1.ObjectMeta{
+				Name:      "myautoscaler",
+				Namespace: metav1.NamespaceDefault,
+			},
+			Spec: autoscaling.HorizontalPodAutoscalerSpec{
+				ScaleTargetRef: autoscaling.CrossVersionObjectReference{
+					Kind: "ReplicationController",
+					Name: "myrc",
+				},
+				MinReplicas: newInt32(1),
+				MaxReplicas: 5,
+				Metrics: []autoscaling.MetricSpec{
+					{
+						Type: autoscaling.ExternalMetricSourceType,
+						External: &autoscaling.ExternalMetricSource{
+							MetricName: "some/metric",
+							MetricSelector: &metav1.LabelSelector{
+								MatchLabels: map[string]string{
+									"label": "value",
+								},
+							},
+							TargetAverageValue: resource.NewMilliQuantity(300, resource.DecimalSI),
+						},
+					},
+				},
+			},
+		},
 	}
 	for _, successCase := range successCases {
 		if errs := ValidateHorizontalPodAutoscaler(&successCase); len(errs) != 0 {
@@ -487,6 +543,130 @@ func TestValidateHorizontalPodAutoscaler(t *testing.T) {
 			},
 			msg: "must specify a metric name",
 		},
+		{
+			horizontalPodAutoscaler: autoscaling.HorizontalPodAutoscaler{
+				ObjectMeta: metav1.ObjectMeta{Name: "myautoscaler", Namespace: metav1.NamespaceDefault},
+				Spec: autoscaling.HorizontalPodAutoscalerSpec{
+					ScaleTargetRef: autoscaling.CrossVersionObjectReference{Name: "myrc", Kind: "ReplicationController"},
+					MinReplicas:    newInt32(1),
+					MaxReplicas:    5,
+					Metrics: []autoscaling.MetricSpec{
+						{
+							Type: autoscaling.ExternalMetricSourceType,
+							External: &autoscaling.ExternalMetricSource{
+								MetricSelector: &metav1.LabelSelector{
+									MatchLabels: map[string]string{
+										"label": "value",
+									},
+								},
+								TargetValue: resource.NewMilliQuantity(300, resource.DecimalSI),
+							},
+						},
+					},
+				},
+			},
+			msg: "must specify a metric name",
+		},
+		{
+			horizontalPodAutoscaler: autoscaling.HorizontalPodAutoscaler{
+				ObjectMeta: metav1.ObjectMeta{Name: "myautoscaler", Namespace: metav1.NamespaceDefault},
+				Spec: autoscaling.HorizontalPodAutoscalerSpec{
+					ScaleTargetRef: autoscaling.CrossVersionObjectReference{Name: "myrc", Kind: "ReplicationController"},
+					MinReplicas:    newInt32(1),
+					MaxReplicas:    5,
+					Metrics: []autoscaling.MetricSpec{
+						{
+							Type: autoscaling.ExternalMetricSourceType,
+							External: &autoscaling.ExternalMetricSource{
+								MetricName: "some/metric",
+								MetricSelector: &metav1.LabelSelector{
+									MatchLabels: map[string]string{
+										"label": "value",
+									},
+								},
+							},
+						},
+					},
+				},
+			},
+			msg: "must set either a target value for metric or a per-pod target",
+		},
+		{
+			horizontalPodAutoscaler: autoscaling.HorizontalPodAutoscaler{
+				ObjectMeta: metav1.ObjectMeta{Name: "myautoscaler", Namespace: metav1.NamespaceDefault},
+				Spec: autoscaling.HorizontalPodAutoscalerSpec{
+					ScaleTargetRef: autoscaling.CrossVersionObjectReference{Name: "myrc", Kind: "ReplicationController"},
+					MinReplicas:    newInt32(1),
+					MaxReplicas:    5,
+					Metrics: []autoscaling.MetricSpec{
+						{
+							Type: autoscaling.ExternalMetricSourceType,
+							External: &autoscaling.ExternalMetricSource{
+								MetricName: "some/metric",
+								MetricSelector: &metav1.LabelSelector{
+									MatchLabels: map[string]string{
+										"label": "value",
+									},
+								},
+								TargetValue: resource.NewMilliQuantity(-300, resource.DecimalSI),
+							},
+						},
+					},
+				},
+			},
+			msg: "must be positive",
+		},
+		{
+			horizontalPodAutoscaler: autoscaling.HorizontalPodAutoscaler{
+				ObjectMeta: metav1.ObjectMeta{Name: "myautoscaler", Namespace: metav1.NamespaceDefault},
+				Spec: autoscaling.HorizontalPodAutoscalerSpec{
+					ScaleTargetRef: autoscaling.CrossVersionObjectReference{Name: "myrc", Kind: "ReplicationController"},
+					MinReplicas:    newInt32(1),
+					MaxReplicas:    5,
+					Metrics: []autoscaling.MetricSpec{
+						{
+							Type: autoscaling.ExternalMetricSourceType,
+							External: &autoscaling.ExternalMetricSource{
+								MetricName: "some/metric",
+								MetricSelector: &metav1.LabelSelector{
+									MatchLabels: map[string]string{
+										"label": "value",
+									},
+								},
+								TargetAverageValue: resource.NewMilliQuantity(-300, resource.DecimalSI),
+							},
+						},
+					},
+				},
+			},
+			msg: "must be positive",
+		},
+		{
+			horizontalPodAutoscaler: autoscaling.HorizontalPodAutoscaler{
+				ObjectMeta: metav1.ObjectMeta{Name: "myautoscaler", Namespace: metav1.NamespaceDefault},
+				Spec: autoscaling.HorizontalPodAutoscalerSpec{
+					ScaleTargetRef: autoscaling.CrossVersionObjectReference{Name: "myrc", Kind: "ReplicationController"},
+					MinReplicas:    newInt32(1),
+					MaxReplicas:    5,
+					Metrics: []autoscaling.MetricSpec{
+						{
+							Type: autoscaling.ExternalMetricSourceType,
+							External: &autoscaling.ExternalMetricSource{
+								MetricName: "some/metric",
+								MetricSelector: &metav1.LabelSelector{
+									MatchLabels: map[string]string{
+										"label": "value",
+									},
+								},
+								TargetValue:        resource.NewMilliQuantity(300, resource.DecimalSI),
+								TargetAverageValue: resource.NewMilliQuantity(300, resource.DecimalSI),
+							},
+						},
+					},
+				},
+			},
+			msg: "may not set both a target value for metric and a per-pod target",
+		},
 		{
 			horizontalPodAutoscaler: autoscaling.HorizontalPodAutoscaler{
 				ObjectMeta: metav1.ObjectMeta{Name: "myautoscaler", Namespace: metav1.NamespaceDefault},