diff --git a/net.go b/net.go
index 17d632d..bde59dc 100644
--- a/net.go
+++ b/net.go
@@ -291,9 +291,13 @@ func (e *EurekaConnection) HeartBeatInstance(ins *Instance) error {
 }
 
 func (i *Instance) Id() string {
+	if i.UniqueID != nil {
+		return i.UniqueID(*i)
+	}
+
 	if i.DataCenterInfo.Name == "Amazon" {
 		return i.DataCenterInfo.Metadata.InstanceID
 	}
-	return i.HostName
 
+	return i.HostName
 }
diff --git a/struct.go b/struct.go
index 0ce6e0b..19b0570 100644
--- a/struct.go
+++ b/struct.go
@@ -98,6 +98,8 @@ type Instance struct {
 
 	LeaseInfo LeaseInfo        `xml:"leaseInfo" json:"leaseInfo"`
 	Metadata  InstanceMetadata `xml:"metadata" json:"metadata"`
+
+	UniqueID func(i Instance) string `xml:"-" json:"-"`
 }
 
 // Port struct used for JSON [un]marshaling only
diff --git a/tests/instance_test.go b/tests/instance_test.go
new file mode 100644
index 0000000..da01ad9
--- /dev/null
+++ b/tests/instance_test.go
@@ -0,0 +1,95 @@
+package fargo_test
+
+// MIT Licensed (see README.md)
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/hudl/fargo"
+	. "github.com/smartystreets/goconvey/convey"
+)
+
+func TestInstanceID(t *testing.T) {
+	i := fargo.Instance{
+		HostName:         "i-6543",
+		Port:             9090,
+		App:              "TESTAPP",
+		IPAddr:           "127.0.0.10",
+		VipAddress:       "127.0.0.10",
+		SecureVipAddress: "127.0.0.10",
+		Status:           fargo.UP,
+	}
+
+	Convey("Given an instance with DataCenterInfo.Name set to Amazon", t, func() {
+		i.DataCenterInfo = fargo.DataCenterInfo{Name: fargo.Amazon}
+
+		Convey("When UniqueID function has NOT been set", func() {
+			i.UniqueID = nil
+
+			Convey("And InstanceID has been set in AmazonMetadata", func() {
+				i.DataCenterInfo.Metadata.InstanceID = "EXPECTED-ID"
+
+				Convey("Id() should return the provided InstanceID", func() {
+					So(i.Id(), ShouldEqual, "EXPECTED-ID")
+				})
+			})
+
+			Convey("And InstanceID has NOT been set in AmazonMetadata", func() {
+				i.DataCenterInfo.Metadata.InstanceID = ""
+
+				Convey("Id() should return an empty string", func() {
+					So(i.Id(), ShouldEqual, "")
+				})
+			})
+		})
+
+		Convey("When UniqueID function has been set", func() {
+			i.UniqueID = func(i fargo.Instance) string {
+				return fmt.Sprintf("%s:%d", i.App, 123)
+			}
+
+			Convey("And InstanceID has been set in AmazonMetadata", func() {
+				i.DataCenterInfo.Metadata.InstanceID = "UNEXPECTED"
+
+				Convey("Id() should return the ID that is provided by UniqueID", func() {
+					So(i.Id(), ShouldEqual, "TESTAPP:123")
+				})
+			})
+
+			Convey("And InstanceID has not been set in AmazonMetadata", func() {
+				i.DataCenterInfo.Metadata.InstanceID = ""
+
+				Convey("Id() should return the ID that is provided by UniqueID", func() {
+					So(i.Id(), ShouldEqual, "TESTAPP:123")
+				})
+			})
+		})
+	})
+
+	Convey("Given an instance with DataCenterInfo.Name set to MyOwn", t, func() {
+		i.DataCenterInfo = fargo.DataCenterInfo{Name: fargo.MyOwn}
+
+		Convey("When UniqueID function has NOT been set", func() {
+			i.UniqueID = nil
+
+			Convey("Id() should return the host name", func() {
+				So(i.Id(), ShouldEqual, "i-6543")
+			})
+		})
+
+		Convey("When UniqueID function has been set", func() {
+			i.Metadata.Raw = []byte(`{"instanceId": "unique-id"}`)
+			i.UniqueID = func(i fargo.Instance) string {
+				if id, err := i.Metadata.GetString("instanceId"); err == nil {
+					return fmt.Sprintf("%s:%s", i.HostName, id)
+				}
+				return i.HostName
+			}
+
+			Convey("Id() should return the ID that is provided by UniqueID", func() {
+				So(i.Id(), ShouldEqual, "i-6543:unique-id")
+			})
+		})
+	})
+}