diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 479caf1..df18575 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -17,10 +17,10 @@ jobs:
     strategy:
       matrix:
         os: [ windows-2022, windows-2025, ubuntu-22.04, ubuntu-24.04 ]
-        php-version: [8.0, 8.1, 8.2, 8.3]
+        php-version: [8.0, 8.1, 8.2, 8.3, 8.4]
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       - name: Setup PHP
         uses: shivammathur/setup-php@v2
@@ -31,7 +31,7 @@ jobs:
         run: composer install
 
       - name: Test
-        run: ./vendor/bin/phpunit tests
+        run: ./vendor/bin/phpunit tests --testdox -v
 
       - uses: Bandwidth/build-notify-slack-action@v2
         if: failure() && !github.event.pull_request.draft
diff --git a/.gitignore b/.gitignore
index f73aa03..c6483f2 100644
--- a/.gitignore
+++ b/.gitignore
@@ -4,6 +4,7 @@ selftests/
 config.php
 /bin
 composer.lock
+composer.phar
 .idea/
 .DS_Store
 .phpunit.result.cache
diff --git a/README.md b/README.md
index caa3326..aafb89b 100644
--- a/README.md
+++ b/README.md
@@ -4,12 +4,13 @@ PHP Client library for Bandwidth's Phone Number Dashboard (AKA: Dashboard, Iris)
 
 ## Supported PHP Versions
 
-| Version | Support Level            |
-|:--------|:-------------------------|
-| 8.0     | Supported |
-| 8.1     | Supported |
-| 8.2     | Supported |
-| 8.3     | Supported |
+| Version | Supported? |
+|:--------|:-----------|
+| 8.0     | Supported  |
+| 8.1     | Supported  |
+| 8.2     | Supported  |
+| 8.3     | Supported  |
+| 8.4     | Supported  |
 
 ## Install
 
@@ -22,8 +23,15 @@ composer require bandwidth/iris
 ## Usage
 
 ```PHP
+// Basic Auth
 $client = new \Iris\Client($login, $password, ['url' => 'https://dashboard.bandwidth.com/api/']);
 
+// Bearer Auth with Token
+$client = new \Iris\Client(null, null, ['accessToken' => '<your_access_token>']);
+
+// Bearer Auth using Client Credentials
+$client = new \Iris\Client(null, null, ['clientId' => '<your_client_id>', 'clientSecret' => '<your_client_secret>']);
+
 ```
 
 ## Run tests
diff --git a/composer.phar b/composer.phar
deleted file mode 100755
index e44722e..0000000
Binary files a/composer.phar and /dev/null differ
diff --git a/core/Client.php b/core/Client.php
index 027bd67..14065c2 100644
--- a/core/Client.php
+++ b/core/Client.php
@@ -135,28 +135,48 @@ final class Client extends iClient
      */
     protected $lastResponseBody = null;
 
+    protected $accessToken = null;
+
+    protected $accessTokenExpiration = null;
+
+    protected $clientId = null;
+
+    protected $clientSecret = null;
+
+    protected $clientOptions = [];
+
     public function __construct($login, $password, $options = [])
     {
-        if (empty($login) || empty($password))
-        {
-            throw new \Exception("Provide login, password");
+        foreach (['accessToken', 'accessTokenExpiration', 'clientId', 'clientSecret'] as $key) {
+            if (array_key_exists($key, $options)) {
+                $this->$key = $options[$key];
+                unset($options[$key]);
+            }
         }
 
-        $options['auth'] = [$login, $password];
-        $options['base_uri'] = $options['url'] ?: 'https://dashboard.bandwidth.com/api';
+        if ($login !== null && $password !== null) {
+            $this->clientOptions['auth'] = [$login, $password];
+        }
+
+        $base = $options['base_uri'] ?? ($options['url'] ?? 'https://dashboard.bandwidth.com/api');
         unset($options['url']);
-        $options['base_uri'] = rtrim($options['base_uri'], '/') . '/';
+        $this->clientOptions['base_uri'] = rtrim($base, '/') . '/';
 
-        $client_options = array();
-        if(isset($options['handler'])) {
-            $client_options['handler'] = $options['handler'];
+        if (isset($options['handler'])) {
+            $this->clientOptions['handler'] = $options['handler'];
+            unset($options['handler']);
+        } else {
+            $this->clientOptions['handler'] = \GuzzleHttp\HandlerStack::create();
         }
 
-        $options['headers'] = array(
-            'User-Agent' => 'PHP-Bandwidth-Iris'
-        );
+        $headers = $options['headers'] ?? [];
+        $headers['User-Agent'] = 'PHP-Bandwidth-Iris';
+        unset($options['headers']);
+        $this->clientOptions['headers'] = $headers;
+
+        $this->clientOptions = array_replace($this->clientOptions, $options);
 
-        $this->client = new \GuzzleHttp\Client($options);
+        $this->client = new \GuzzleHttp\Client($this->clientOptions);
     }
 
     /**
@@ -289,6 +309,19 @@ public function request($method, $url, $options = [], $parse = true)
         $this->lastResponseBody = null;
         try
         {
+            // Get token if we don't have one, or if it's expired, and we have client credentials
+            if ((empty($this->accessToken) || (!empty($this->accessTokenExpiration) && $this->accessTokenExpiration <= time() + 60))
+                && !empty($this->clientId) && !empty($this->clientSecret)) {
+                $this->fetchAccessToken();
+            }
+
+            // If we have a valid access token, add it to the request headers
+            if (!empty($this->accessToken) && (empty($this->accessTokenExpiration) || $this->accessTokenExpiration > time() + 60)) {
+                $options['headers'] = $options['headers'] ?? [];
+                if (empty($options['headers']['Authorization'])) {
+                    $options['headers']['Authorization'] = 'Bearer ' . $this->accessToken;
+                }
+            }
             $response = $this->client->request($method, ltrim($url, '/'), $options);
             if (!$parse)
             {
@@ -302,6 +335,23 @@ public function request($method, $url, $options = [], $parse = true)
         }
     }
 
+    private function fetchAccessToken()
+    {
+        $tokenUrl = 'https://api.bandwidth.com/api/v1/oauth2/token';
+        $tokenOptions = [
+            'auth' => [$this->clientId, $this->clientSecret],
+            'form_params' => [
+                'grant_type' => 'client_credentials'
+            ],
+        ];
+        $oauthClient = new \GuzzleHttp\Client($this->clientOptions);
+
+        $tokenResponse = $oauthClient->request('post', $tokenUrl, $tokenOptions);
+        $tokenData = json_decode((string)$tokenResponse->getBody(), true);
+        $this->accessToken = $tokenData['access_token'] ?? null;
+        $this->accessTokenExpiration = isset($tokenData['expires_in']) ? (time() + (int)$tokenData['expires_in']) : null;
+    }
+
     /**
      * Returns the XML string received in the last response.
      * @return string $lastResponseBody
diff --git a/tests/OAuthTest.php b/tests/OAuthTest.php
new file mode 100644
index 0000000..a04a8b2
--- /dev/null
+++ b/tests/OAuthTest.php
@@ -0,0 +1,96 @@
+<?php
+use GuzzleHttp\Handler\MockHandler;
+use GuzzleHttp\HandlerStack;
+use GuzzleHttp\Psr7\Response;
+use GuzzleHttp\Middleware;
+
+use PHPUnit\Framework\TestCase;
+
+class OAuthTest extends TestCase {
+    private const API_URL = 'https://api.test.com/v1.0';
+    private const INSERVICE_URL = self::API_URL . '/accounts/9500249/inserviceNumbers';
+    private const TOKEN_URL = 'https://api.bandwidth.com/api/v1/oauth2/token';
+
+    private const INSERVICE_RESPONSE = "<?xml version=\"1.0\" encoding=\"UTF-8\" standalone=\"yes\"?><TNs><TotalCount>1</TotalCount><Links><first>link</first></Links><TelephoneNumbers><Count>1</Count><TelephoneNumber>8043024183</TelephoneNumber></TelephoneNumbers></TNs>";
+    private const TOKEN_RESPONSE = '{"access_token":"abcdef123456","expires_in":3600}';
+
+    private const TOKEN_REQUEST_AUTH_STRING = 'Basic Y2xpZW50X2lkOmNsaWVudF9zZWNyZXQ=';
+    private const BEARER_AUTH_STRING = 'Bearer abcdef123456';
+
+    private function makeAccount(array $responses, array $clientOptions, array &$container, ?string $login = null, ?string $password = null): Iris\Account {
+        $mock = new MockHandler($responses);
+        $handler = HandlerStack::create($mock);
+        $history = Middleware::history($container);
+        $handler->push($history);
+        $client = new Iris\Client($login, $password, array_merge(['url' => self::API_URL, 'handler' => $handler], $clientOptions));
+        return new Iris\Account(9500249, $client);
+    }
+
+    private function assertRequest(array $container, int $idx, string $method, string $uri, ?string $authHeader = null): void {
+        $request = $container[$idx]['request'];
+        $this->assertSame($uri, (string)$request->getUri());
+        $this->assertSame($method, $request->getMethod());
+        if ($authHeader !== null) {
+            $this->assertSame($authHeader, $request->getHeaderLine('Authorization'));
+        }
+    }
+
+    public function testBasicAuth(): void {
+        $container = [];
+        $account = $this->makeAccount([
+            new Response(200, [], self::INSERVICE_RESPONSE),
+        ], [], $container, 'username', 'password');
+
+        $account->inserviceNumbers();
+        $this->assertRequest($container, 0, 'GET', self::INSERVICE_URL, 'Basic ' . base64_encode('username:password'));
+    }
+
+    public function testOAuth(): void {
+        $container = [];
+        $account = $this->makeAccount([
+            new Response(200, [], self::TOKEN_RESPONSE),
+            new Response(200, [], self::INSERVICE_RESPONSE),
+            new Response(200, [], self::INSERVICE_RESPONSE),
+        ], [
+            'clientId' => 'client_id',
+            'clientSecret' => 'client_secret',
+        ], $container);
+
+        $account->inserviceNumbers();
+        $account->inserviceNumbers();
+
+        $this->assertRequest($container, 0, 'POST', self::TOKEN_URL, self::TOKEN_REQUEST_AUTH_STRING);
+        $this->assertRequest($container, 1, 'GET', self::INSERVICE_URL, self::BEARER_AUTH_STRING);
+        $this->assertRequest($container, 2, 'GET', self::INSERVICE_URL, self::BEARER_AUTH_STRING);
+    }
+
+    public function testToken(): void {
+        $container = [];
+        $account = $this->makeAccount([
+            new Response(200, [], self::INSERVICE_RESPONSE),
+        ], [
+            'accessToken' => 'access_token',
+            'accessTokenExpiration' => time() + 3600,
+        ], $container);
+
+        $account->inserviceNumbers();
+        $this->assertRequest($container, 0, 'GET', self::INSERVICE_URL, 'Bearer access_token');
+    }
+
+    public function testExpiredToken(): void {
+        $container = [];
+        $account = $this->makeAccount([
+            new Response(200, [], self::TOKEN_RESPONSE),
+            new Response(200, [], self::INSERVICE_RESPONSE),
+        ], [
+            'accessToken' => 'expired_token',
+            'accessTokenExpiration' => time() - 3600,
+            'clientId' => 'client_id',
+            'clientSecret' => 'client_secret',
+        ], $container);
+
+        $account->inserviceNumbers();
+        $this->assertRequest($container, 0, 'POST', self::TOKEN_URL, self::TOKEN_REQUEST_AUTH_STRING);
+        $this->assertRequest($container, 1, 'GET', self::INSERVICE_URL, self::BEARER_AUTH_STRING);
+    }
+}