diff --git a/google/downscoped/downscoping.go b/google/downscoped/downscoping.go
new file mode 100644
index 0000000..5d2ce6e
--- /dev/null
+++ b/google/downscoped/downscoping.go
@@ -0,0 +1,116 @@
+package downscoped
+
+import (
+	"encoding/json"
+	"fmt"
+	"golang.org/x/oauth2"
+	"io/ioutil"
+	"net/http"
+	"net/url"
+	"time"
+)
+
+const (
+	IDENTITY_BINDING_ENDPOINT = "https://sts.googleapis.com/v1beta/token"
+)
+
+// Defines an upper bound of permissions available for a GCP credential for one or more resources
+type AccessBoundary struct {
+	AccessBoundaryRules []AccessBoundaryRule `json:"accessBoundaryRules"`
+}
+
+func NewAccessBoundary() AccessBoundary {
+	return AccessBoundary{make([]AccessBoundaryRule, 0)}
+}
+
+type AvailabilityCondition struct {
+	Title       string `json:"title,omitempty"`
+	Expression  string `json:"expression"`
+	Description string `json:"description,omitempty"`
+}
+
+type AccessBoundaryRule struct {
+	AvailableResource    string                 `json:"availableResource"`
+	AvailablePermissions []string               `json:"availablePermissions"`
+	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"`
+}
+
+type DownscopingConfig struct {
+	RootSource               oauth2.TokenSource
+	CredentialAccessBoundary AccessBoundary
+}
+
+func DownscopedTokenWithEndpoint(config DownscopingConfig, endpoint string) (oauth2.TokenSource, error) {
+	if config.RootSource == nil {
+		return nil, fmt.Errorf("oauth2/google: rootTokenSource cannot be nil")
+	}
+	if len(config.CredentialAccessBoundary.AccessBoundaryRules) == 0 {
+		return nil, fmt.Errorf("oauth2/google: length of AccessBoundaryRules must be at least 1")
+	}
+
+	downscopedOptions := struct {
+		Boundary AccessBoundary `json:"accessBoundary"`
+	}{
+		Boundary: config.CredentialAccessBoundary,
+	}
+
+	tok, err := config.RootSource.Token()
+	if err != nil {
+		return nil, fmt.Errorf("oauth2/google: unable to refresh root token %v", err)
+	}
+
+	b, err := json.Marshal(downscopedOptions) // TODO: make sure that this marshals properly!
+	if err != nil {
+		return nil, fmt.Errorf("oauth2/google: Unable to marshall 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", url.QueryEscape(string(b)))
+
+	resp, err := http.PostForm(endpoint, form)
+	defer resp.Body.Close()
+	if err != nil {
+		return nil, fmt.Errorf("unable to generate POST Request %v", err)
+	}
+
+	if resp.StatusCode != http.StatusOK {
+		bodyBytes, _ := ioutil.ReadAll(resp.Body)
+		return nil, fmt.Errorf("unable to exchange token %v", string(bodyBytes))
+	}
+
+	tresp := DownScopedTokenResponse{}
+	json.NewDecoder(resp.Body).Decode(&tresp)
+
+	// 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 expiry_time time.Time
+	if tresp.ExpiresIn > 0 {
+		expiry_time = time.Now().Add(time.Duration(time.Duration(tresp.ExpiresIn) * time.Second))
+	} else {
+		expiry_time = tok.Expiry
+	}
+
+	newToken := &oauth2.Token{
+		AccessToken: tresp.AccessToken,
+		TokenType:   tresp.TokenType,
+		Expiry:      expiry_time,
+	}
+	return oauth2.StaticTokenSource(newToken), nil
+}
+
+func NewDownscopedTokenSource(config DownscopingConfig) (oauth2.TokenSource, error) {
+	return DownscopedTokenWithEndpoint(config, IDENTITY_BINDING_ENDPOINT)
+}
diff --git a/google/downscoped/downscoping_test.go b/google/downscoped/downscoping_test.go
new file mode 100644
index 0000000..fca1601
--- /dev/null
+++ b/google/downscoped/downscoping_test.go
@@ -0,0 +1,55 @@
+package downscoped
+
+import (
+	"golang.org/x/oauth2"
+	"io/ioutil"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+var (
+	standardReqBody  = "grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Atoken-exchange&options=%257B%2522accessBoundary%2522%253A%257B%2522accessBoundaryRules%2522%253A%255B%257B%2522availableResource%2522%253A%2522test1%2522%252C%2522availablePermissions%2522%253A%255B%2522Perm1%252C%2Bperm2%2522%255D%257D%255D%257D%257D&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_NewAccessBoundary(t *testing.T) {
+	got := NewAccessBoundary()
+	want := AccessBoundary{nil}
+	if got.AccessBoundaryRules == nil || len(got.AccessBoundaryRules) != 0 {
+		t.Errorf("NewAccessBoundary() = %v; want %v", got, want)
+	}
+}
+
+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() != "/" { //TODO: Will this work, or do I need to redirect this to this test server instead?
+			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 := NewAccessBoundary()
+	new.AccessBoundaryRules = append(new.AccessBoundaryRules, AccessBoundaryRule{"test1", []string{"Perm1, perm2"}, nil})
+	myTok := oauth2.Token{AccessToken: "Mellon"}
+	tmpSrc := oauth2.StaticTokenSource(&myTok)
+	out, err := DownscopedTokenWithEndpoint(DownscopingConfig{tmpSrc, new}, ts.URL)
+	if err != nil {
+		t.Fatalf("NewDownscopedTokenSource failed with error: %v", err)
+	}
+	_, err = out.Token()
+	if err != nil {
+		t.Fatalf("Token() call failed with error %v", err)
+	}
+}