diff --git a/google/downscope/downscoping.go b/google/downscope/downscoping.go
new file mode 100644
index 0000000..2d74c37
--- /dev/null
+++ b/google/downscope/downscoping.go
@@ -0,0 +1,190 @@
+// Copyright 2021 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 downscope implements the ability to downscope, or restrict, the
+Identity and AccessManagement permissions that a short-lived Token
+can use. Please note that only Google Cloud Storage supports this feature.
+For complete documentation, see https://cloud.google.com/iam/docs/downscoping-short-lived-credentials
+*/
+package downscope
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"io/ioutil"
+	"net/http"
+	"net/url"
+	"time"
+
+	"golang.org/x/oauth2"
+)
+
+var (
+	identityBindingEndpoint = "https://sts.googleapis.com/v1/token"
+)
+
+type accessBoundary struct {
+	AccessBoundaryRules []AccessBoundaryRule `json:"accessBoundaryRules"`
+}
+
+// An AvailabilityCondition restricts access to a given Resource.
+type AvailabilityCondition struct {
+	// An Expression specifies the Cloud Storage objects where
+	// permissions are available. For further documentation, see
+	// https://cloud.google.com/iam/docs/conditions-overview
+	Expression string `json:"expression"`
+	// Title is short string that identifies the purpose of the condition. Optional.
+	Title string `json:"title,omitempty"`
+	// Description details about the purpose of the condition. Optional.
+	Description string `json:"description,omitempty"`
+}
+
+// An AccessBoundaryRule Sets the permissions (and optionally conditions)
+// that the new token has on given resource.
+type AccessBoundaryRule struct {
+	// AvailableResource is the full resource name of the Cloud Storage bucket that the rule applies to.
+	// Use the format //storage.googleapis.com/projects/_/buckets/bucket-name.
+	AvailableResource string `json:"availableResource"`
+	// AvailablePermissions is a list that defines the upper bound on the available permissions
+	// for the resource. Each value is the identifier for an IAM predefined role or custom role,
+	// with the prefix inRole:. For example: inRole:roles/storage.objectViewer.
+	// Only the permissions in these roles will be available.
+	AvailablePermissions []string `json:"availablePermissions"`
+	// An Condition restricts the availability of permissions
+	// to specific Cloud Storage objects. Optional.
+	//
+	// A Condition can be used  to make permissions available for specific objects,
+	// rather than all objects in a Cloud Storage bucket.
+	Condition *AvailabilityCondition `json:"availabilityCondition,omitempty"`
+}
+
+type downscopedTokenResponse struct {
+	AccessToken     string `json:"access_token"`
+	IssuedTokenType string `json:"issued_token_type"`
+	TokenType       string `json:"token_type"`
+	ExpiresIn       int    `json:"expires_in"`
+}
+
+// DownscopingConfig specifies the information necessary to request a downscoped token.
+type DownscopingConfig struct {
+	// RootSource is the TokenSource used to create the downscoped token.
+	// The downscoped token therefore has some subset of the accesses of
+	// the original RootSource.
+	RootSource oauth2.TokenSource
+	// Rules defines the accesses held by the new
+	// downscoped Token. One or more AccessBoundaryRules are required to
+	// define permissions for the new downscoped token. Each one defines an
+	// access (or set of accesses) that the new token has to a given resource.
+	// There can be a maximum of 10 AccessBoundaryRules.
+	Rules []AccessBoundaryRule
+}
+
+// A downscopingTokenSource is used to retrieve a downscoped token with restricted
+// permissions compared to the root Token that is used to generate it.
+type downscopingTokenSource struct {
+	// ctx is the context used to query the API to retrieve a downscoped Token.
+	ctx context.Context
+	// config holds the information necessary to generate a downscoped Token.
+	config DownscopingConfig
+}
+
+// NewTokenSource returns an empty downscopingTokenSource.
+func NewTokenSource(ctx context.Context, conf DownscopingConfig) (oauth2.TokenSource, error) {
+	if conf.RootSource == nil {
+		return nil, fmt.Errorf("downscope: rootSource cannot be nil")
+	}
+	if len(conf.Rules) == 0 {
+		return nil, fmt.Errorf("downscope: length of AccessBoundaryRules must be at least 1")
+	}
+	if len(conf.Rules) > 10 {
+		return nil, fmt.Errorf("downscope: length of AccessBoundaryRules may not be greater than 10")
+	}
+	for _, val := range conf.Rules {
+		if val.AvailableResource == "" {
+			return nil, fmt.Errorf("downscope: all rules must have a nonempty AvailableResource: %+v", val)
+		}
+		if len(val.AvailablePermissions) == 0 {
+			return nil, fmt.Errorf("downscope: all rules must provide at least one permission: %+v", val)
+		}
+	}
+	return downscopingTokenSource{ctx: ctx, config: conf}, nil
+}
+
+// Token() uses a downscopingTokenSource to generate an oauth2 Token.
+// Do note that the returned TokenSource is an oauth2.StaticTokenSource. If you wish
+// to refresh this token automatically, then initialize a locally defined
+// TokenSource struct with the Token held by the StaticTokenSource and wrap
+// that TokenSource in an oauth2.ReuseTokenSource.
+func (dts downscopingTokenSource) Token() (*oauth2.Token, error) {
+
+	downscopedOptions := struct {
+		Boundary accessBoundary `json:"accessBoundary"`
+	}{
+		Boundary: accessBoundary{
+			AccessBoundaryRules: dts.config.Rules,
+		},
+	}
+
+	tok, err := dts.config.RootSource.Token()
+	if err != nil {
+		return nil, fmt.Errorf("downscope: unable to obtain root token: %v", err)
+	}
+
+	b, err := json.Marshal(downscopedOptions)
+	if err != nil {
+		return nil, fmt.Errorf("downscope: unable to marshal AccessBoundary payload %v", err)
+	}
+
+	form := url.Values{}
+	form.Add("grant_type", "urn:ietf:params:oauth:grant-type:token-exchange")
+	form.Add("subject_token_type", "urn:ietf:params:oauth:token-type:access_token")
+	form.Add("requested_token_type", "urn:ietf:params:oauth:token-type:access_token")
+	form.Add("subject_token", tok.AccessToken)
+	form.Add("options", string(b))
+
+	myClient := oauth2.NewClient(dts.ctx, nil)
+	resp, err := myClient.PostForm(identityBindingEndpoint, form)
+	if err != nil {
+		return nil, fmt.Errorf("unable to generate POST Request %v", err)
+	}
+	defer resp.Body.Close()
+	respBody, err := ioutil.ReadAll(resp.Body)
+	if err != nil {
+		return nil, fmt.Errorf("downscope: unable to read reaponse body: %v", err)
+	}
+	if resp.StatusCode != http.StatusOK {
+		b, err := ioutil.ReadAll(resp.Body)
+		if err != nil {
+			return nil, fmt.Errorf("downscope: unable to exchange token; %v.  Failed to read response body: %v", resp.StatusCode, err)
+		}
+		return nil, fmt.Errorf("downscope: unable to exchange token; %v.  Server responsed: %v", resp.StatusCode, string(b))
+	}
+
+	var tresp downscopedTokenResponse
+
+	err = json.Unmarshal(respBody, &tresp)
+	if err != nil {
+		return nil, fmt.Errorf("downscope: unable to unmarshal response body: %v", err)
+	}
+
+	// an exchanged token that is derived from a service account (2LO) has an expired_in value
+	// a token derived from a users token (3LO) does not.
+	// The following code uses the time remaining on rootToken for a user as the value for the
+	// derived token's lifetime
+	var expiryTime time.Time
+	if tresp.ExpiresIn > 0 {
+		expiryTime = time.Now().Add(time.Duration(tresp.ExpiresIn) * time.Second)
+	} else {
+		expiryTime = tok.Expiry
+	}
+
+	newToken := &oauth2.Token{
+		AccessToken: tresp.AccessToken,
+		TokenType:   tresp.TokenType,
+		Expiry:      expiryTime,
+	}
+	return newToken, nil
+}
diff --git a/google/downscope/downscoping_test.go b/google/downscope/downscoping_test.go
new file mode 100644
index 0000000..d5adda1
--- /dev/null
+++ b/google/downscope/downscoping_test.go
@@ -0,0 +1,55 @@
+// Copyright 2021 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 downscope
+
+import (
+	"context"
+	"io/ioutil"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"golang.org/x/oauth2"
+)
+
+var (
+	standardReqBody  = "grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Atoken-exchange&options=%7B%22accessBoundary%22%3A%7B%22accessBoundaryRules%22%3A%5B%7B%22availableResource%22%3A%22test1%22%2C%22availablePermissions%22%3A%5B%22Perm1%22%2C%22Perm2%22%5D%7D%5D%7D%7D&requested_token_type=urn%3Aietf%3Aparams%3Aoauth%3Atoken-type%3Aaccess_token&subject_token=Mellon&subject_token_type=urn%3Aietf%3Aparams%3Aoauth%3Atoken-type%3Aaccess_token"
+	standardRespBody = `{"access_token":"Open Sesame","expires_in":432,"issued_token_type":"urn:ietf:params:oauth:token-type:access_token","token_type":"Bearer"}`
+)
+
+func Test_DownscopedTokenSource(t *testing.T) {
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.Method != "POST" {
+			t.Errorf("Unexpected request method, %v is found", r.Method)
+		}
+		if r.URL.String() != "/" {
+			t.Errorf("Unexpected request URL, %v is found", r.URL)
+		}
+		body, err := ioutil.ReadAll(r.Body)
+		if err != nil {
+			t.Fatalf("Failed to read request body: %v", err)
+		}
+		if got, want := string(body), standardReqBody; got != want {
+			t.Errorf("Unexpected exchange payload: got %v but want %v,", got, want)
+		}
+		w.Header().Set("Content-Type", "application/json")
+		w.Write([]byte(standardRespBody))
+
+	}))
+	new := []AccessBoundaryRule{
+		{
+			AvailableResource:    "test1",
+			AvailablePermissions: []string{"Perm1", "Perm2"},
+		},
+	}
+	myTok := oauth2.Token{AccessToken: "Mellon"}
+	tmpSrc := oauth2.StaticTokenSource(&myTok)
+	dts := downscopingTokenSource{context.Background(), DownscopingConfig{tmpSrc, new}}
+	identityBindingEndpoint = ts.URL
+	_, err := dts.Token()
+	if err != nil {
+		t.Fatalf("NewDownscopedTokenSource failed with error: %v", err)
+	}
+}
diff --git a/google/downscope/example_test.go b/google/downscope/example_test.go
new file mode 100644
index 0000000..061cf57
--- /dev/null
+++ b/google/downscope/example_test.go
@@ -0,0 +1,38 @@
+// Copyright 2021 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 downscope_test
+
+import (
+	"context"
+
+	"golang.org/x/oauth2"
+	"golang.org/x/oauth2/google/downscope"
+)
+
+func ExampleNewTokenSource() {
+	ctx := context.Background()
+	// Initializes an accessBoundary with one Rule.
+	accessBoundary := []downscope.AccessBoundaryRule{
+		{
+			AvailableResource:    "//storage.googleapis.com/projects/_/buckets/foo",
+			AvailablePermissions: []string{"inRole:roles/storage.objectViewer"},
+		},
+	}
+
+	var rootSource oauth2.TokenSource
+	// This Source can be initialized in multiple ways; the following example uses
+	// Application Default Credentials.
+
+	// rootSource, err := google.DefaultTokenSource(ctx, "https://www.googleapis.com/auth/cloud-platform")
+
+	dts, err := downscope.NewTokenSource(ctx, downscope.DownscopingConfig{RootSource: rootSource, Rules: accessBoundary})
+	if err != nil {
+		_ = dts
+	}
+	// You can now use the token held in myTokenSource to make
+	// Google Cloud Storage calls, as follows:
+
+	// storageClient, err := storage.NewClient(ctx, option.WithTokenSource(myTokenSource))
+}