diff --git a/client.go b/client.go
index b494af8..ee78652 100644
--- a/client.go
+++ b/client.go
@@ -125,10 +125,15 @@ func (c *Client) Send(req *http.Request, v interface{}) error {
 // making the main request
 // client.Token will be updated when changed
 func (c *Client) SendWithAuth(req *http.Request, v interface{}) error {
+	c.Lock()
+	// Note: Here we do not want to `defer c.Unlock()` because we need `c.Send(...)`
+	// to happen outside of the locked section.
+
 	if c.Token != nil {
 		if !c.tokenExpiresAt.IsZero() && c.tokenExpiresAt.Sub(time.Now()) < RequestNewTokenBeforeExpiresIn {
 			// c.Token will be updated in GetAccessToken call
 			if _, err := c.GetAccessToken(); err != nil {
+				c.Unlock()
 				return err
 			}
 		}
@@ -136,6 +141,9 @@ func (c *Client) SendWithAuth(req *http.Request, v interface{}) error {
 		req.Header.Set("Authorization", "Bearer "+c.Token.Token)
 	}
 
+	// Unlock the client mutex before sending the request, this allows multiple requests
+	// to be in progress at the same time.
+	c.Unlock()
 	return c.Send(req, v)
 }
 
diff --git a/types.go b/types.go
index 2eb4c82..6441ddf 100644
--- a/types.go
+++ b/types.go
@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"io"
 	"net/http"
+	"sync"
 	"time"
 )
 
@@ -168,6 +169,7 @@ type (
 
 	// Client represents a Paypal REST API Client
 	Client struct {
+		sync.Mutex
 		Client         *http.Client
 		ClientID       string
 		Secret         string