diff --git a/google/example_test.go b/google/example_test.go
index 643f50716..bdba9e5fa 100644
--- a/google/example_test.go
+++ b/google/example_test.go
@@ -5,12 +5,13 @@
 package google_test
 
 import (
+	"context"
 	"fmt"
 	"io/ioutil"
 	"log"
 	"net/http"
+	"time"
 
-	"golang.org/x/net/context"
 	"golang.org/x/oauth2"
 	"golang.org/x/oauth2/google"
 	"golang.org/x/oauth2/jwt"
@@ -160,3 +161,32 @@ func ExampleCredentialsFromJSON() {
 	}
 	_ = creds // TODO: Use creds.
 }
+
+func ExampleImpersonatedCredentials() {
+	ctx := context.Background()
+	targetPrincipal := "impersonated-account@project.iam.gserviceaccount.com"
+	lifetime := 30 * time.Second
+	delegates := []string{}
+	targetScopes := []string{"https://www.googleapis.com/auth/devstorage.read_only"}
+	rootTokenSource, err := google.DefaultTokenSource(ctx,
+		"https://www.googleapis.com/auth/iam")
+	if err != nil {
+		log.Fatal(err)
+	}
+	impersonatedTokenSource, err := google.ImpersonatedTokenSource(
+		google.ImpersonatedTokenConfig{
+			RootTokenSource: rootTokenSource,
+			TargetPrincipal: targetPrincipal,
+			Lifetime:        lifetime,
+			Delegates:       delegates,
+			TargetScopes:    targetScopes,
+		},
+	)
+
+	client := &http.Client{
+		Transport: &oauth2.Transport{
+			Source: impersonatedTokenSource,
+		},
+	}
+	client.Get("...")
+}
diff --git a/google/impersonate.go b/google/impersonate.go
new file mode 100644
index 000000000..80d0fbf92
--- /dev/null
+++ b/google/impersonate.go
@@ -0,0 +1,126 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package google
+
+import (
+	"context"
+	"fmt"
+	"sync"
+	"time"
+
+	"golang.org/x/oauth2"
+	"google.golang.org/api/iamcredentials/v1"
+)
+
+// ImpersonatedTokenConfig prameters to start Credential impersonation exchange.
+type ImpersonatedTokenConfig struct {
+	RootTokenSource oauth2.TokenSource
+	TargetPrincipal string
+	Lifetime        time.Duration
+	Delegates       []string
+	TargetScopes    []string
+}
+
+// ImpersonatedTokenSource returns a TokenSource issued to a user or
+// service account to impersonate another. The source project using
+// ImpersonatedTokenSource must enable the "IAMCredentials" API.  Also, the
+// target service account must grant the orginating principal the
+// "Service Account Token Creator" IAM role:
+// https://cloud.google.com/iam/docs/service-accounts#the_service_account_token_creator_role
+//
+//  rootSource (TokenSource): The root TokenSource
+//     used as to acquire the target identity TokenSource.
+//     rootSource *must* include scopes that contains
+//     "https://www.googleapis.com/auth/iam"
+//     or
+//     "https://www.googleapis.com/auth/cloud.platform"
+//  targetPrincipal (string): The service account to impersonate.
+//  targetScopes ([]string): Scopes to request during the
+//     authorization grant.
+//  delegates ([]string): The chained list of delegates required
+//      to grant the final access_token. If set, the sequence of
+//      identities must have "Service Account Token Creator" capability
+//      granted to the preceeding identity. For example, if set to
+//      [serviceAccountB, serviceAccountC], the source_credential
+//      must have the Token Creator role on serviceAccountB.
+//      serviceAccountB must have the Token Creator on serviceAccountC.
+//      Finally, C must have Token Creator on target_principal.
+//      If left unset, source_credential must have that role on
+//      target_principal.
+//  lifetime (time.Duration): Number of seconds the impersonated credential should
+//     be valid for (up to 3600).
+//
+// Note that this is not a standard OAuth flow, but rather uses Google Cloud
+// IAMCredentials API to exchange one oauth token for an impersonated account
+// see: https://cloud.google.com/iam/credentials/reference/rest/v1/projects.serviceAccounts/generateAccessToken
+func ImpersonatedTokenSource(tokenConfig ImpersonatedTokenConfig) (oauth2.TokenSource, error) {
+
+	if tokenConfig.RootTokenSource == nil {
+		return nil, fmt.Errorf("oauth2/google: rootSource cannot be nil")
+	}
+	if tokenConfig.Lifetime > (3600 * time.Second) {
+		return nil, fmt.Errorf("oauth2/google: lifetime must be less than or equal to 3600 seconds")
+	}
+
+	return &impersonatedTokenSource{
+		refreshMutex:      &sync.Mutex{}, // guards impersonatedToken; held while fetching or updating it.
+		impersonatedToken: nil,           // Token representing the impersonated identity. Initially nil.
+
+		rootSource:      tokenConfig.RootTokenSource,
+		targetPrincipal: tokenConfig.TargetPrincipal,
+		lifetime:        tokenConfig.Lifetime,
+		delegates:       tokenConfig.Delegates,
+		targetScopes:    tokenConfig.TargetScopes,
+	}, nil
+}
+
+type impersonatedTokenSource struct {
+	refreshMutex      *sync.Mutex   // guards impersonatedToken; held while fetching or updating it.
+	impersonatedToken *oauth2.Token // Token representing the impersonated identity.
+
+	rootSource      oauth2.TokenSource
+	targetPrincipal string
+	lifetime        time.Duration
+	delegates       []string
+	targetScopes    []string
+}
+
+func (ts *impersonatedTokenSource) Token() (*oauth2.Token, error) {
+
+	ts.refreshMutex.Lock()
+	defer ts.refreshMutex.Unlock()
+
+	if ts.impersonatedToken.Valid() {
+		return ts.impersonatedToken, nil
+	}
+	client := oauth2.NewClient(context.TODO(), ts.rootSource)
+
+	service, err := iamcredentials.New(client)
+	if err != nil {
+		return nil, fmt.Errorf("oauth2/google: Error creating IAMCredentials: %v", err)
+	}
+	name := fmt.Sprintf("projects/-/serviceAccounts/%s", ts.targetPrincipal)
+	tokenRequest := &iamcredentials.GenerateAccessTokenRequest{
+		Lifetime:  fmt.Sprintf("%ds", int(ts.lifetime.Seconds())),
+		Delegates: ts.delegates,
+		Scope:     ts.targetScopes,
+	}
+	at, err := service.Projects.ServiceAccounts.GenerateAccessToken(name, tokenRequest).Do()
+	if err != nil {
+		return nil, fmt.Errorf("oauth2/google: Error calling iamcredentials.GenerateAccessToken: %v", err)
+	}
+
+	expireAt, err := time.Parse(time.RFC3339, at.ExpireTime)
+	if err != nil {
+		return nil, fmt.Errorf("oauth2/google: Error parsing ExpireTime from iamcredentials: %v", err)
+	}
+
+	ts.impersonatedToken = &oauth2.Token{
+		AccessToken: at.AccessToken,
+		Expiry:      expireAt,
+	}
+
+	return ts.impersonatedToken, nil
+}