diff --git a/.gitattributes b/.gitattributes
index aa6c312..21be40c 100644
--- a/.gitattributes
+++ b/.gitattributes
@@ -2,4 +2,5 @@
 /.github/ export-ignore
 /.gitignore export-ignore
 /phpunit.xml.dist export-ignore
+/phpunit.xml.legacy export-ignore
 /tests/ export-ignore
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 559f37d..13eaf46 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -11,6 +11,8 @@ jobs:
     strategy:
       matrix:
         php:
+          - 8.1
+          - 8.0
           - 7.4
           - 7.3
           - 7.2
@@ -27,3 +29,6 @@ jobs:
           coverage: xdebug
       - run: composer install
       - run: vendor/bin/phpunit --coverage-text
+        if: ${{ matrix.php >= 7.3 }}
+      - run: vendor/bin/phpunit --coverage-text -c phpunit.xml.legacy
+        if: ${{ matrix.php < 7.3 }}
diff --git a/README.md b/README.md
index 3355aab..77a0774 100644
--- a/README.md
+++ b/README.md
@@ -8,8 +8,6 @@ It is heavily influenced by [async.js](https://github.com/caolan/async).
 
 [![CI status](https://github.com/reactphp/async/workflows/CI/badge.svg)](https://github.com/reactphp/async/actions)
 
-## About
-
 This library allows you to manage async control flow. It provides a number of
 combinators for continuation-passing style (aka callbacks). Instead of nesting
 those callbacks, you can declare them as a list, which is resolved
@@ -24,20 +22,17 @@ loop, it can be used with this library.
 *You must be running inside an event loop for react/async to make any sense
 whatsoever!*
 
-## Install
+**Table of Contents**
 
-The recommended way to install react/async is [through
-composer](http://getcomposer.org).
+* [Usage](#usage)
+    * [Parallel](#parallel)
+    * [Waterfall](#waterfall)
+* [Todo](#todo)
+* [Install](#install)
+* [Tests](#tests)
+* [License](#license)
 
-```JSON
-{
-    "require": {
-        "react/async": "~1.0"
-    }
-}
-```
-
-## Example
+## Usage
 
 ### Parallel
 
@@ -110,6 +105,24 @@ Async::waterfall(array(
 
  * Implement queue()
 
+## Install
+
+The recommended way to install this library is [through Composer](https://getcomposer.org/).
+[New to Composer?](https://getcomposer.org/doc/00-intro.md)
+
+Once released, this project will follow [SemVer](https://semver.org/).
+At the moment, this will install the latest development version:
+
+```bash
+$ composer require react/async:dev-main
+```
+
+See also the [CHANGELOG](CHANGELOG.md) for details about version upgrades.
+
+This project aims to run on any platform and thus does not require any PHP
+extensions and supports running on legacy PHP 5.3 through current PHP 8+.
+It's *highly recommended to use the latest supported PHP version* for this project.
+
 ## Tests
 
 To run the test suite, you first need to clone this repo and then install all
diff --git a/composer.json b/composer.json
index fc685f7..d864160 100644
--- a/composer.json
+++ b/composer.json
@@ -29,7 +29,7 @@
         "php": ">=5.3.2"
     },
     "require-dev": {
-        "phpunit/phpunit": "^5.7 || ^4.8.35",
+        "phpunit/phpunit": "^9.3 || ^5.7 || ^4.8.35",
         "react/event-loop": "^1.2"
     },
     "suggest": {
diff --git a/phpunit.xml.dist b/phpunit.xml.dist
index 49eb132..fa88e7e 100644
--- a/phpunit.xml.dist
+++ b/phpunit.xml.dist
@@ -1,25 +1,19 @@
 <?xml version="1.0" encoding="UTF-8"?>
 
-<phpunit backupGlobals="false"
-         backupStaticAttributes="false"
-         colors="true"
-         convertErrorsToExceptions="true"
-         convertNoticesToExceptions="true"
-         convertWarningsToExceptions="true"
-         processIsolation="false"
-         stopOnFailure="false"
-         syntaxCheck="false"
+<!-- PHPUnit configuration file with new format for PHPUnit 9.3+ -->
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
+         xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/9.3/phpunit.xsd"
          bootstrap="vendor/autoload.php"
->
+         colors="true"
+         cacheResult="false">
     <testsuites>
-        <testsuite name="React-Async Test Suite">
+        <testsuite name="React Test Suite">
             <directory>./tests/</directory>
         </testsuite>
     </testsuites>
-
-    <filter>
-        <whitelist>
+    <coverage>
+        <include>
             <directory>./src/</directory>
-        </whitelist>
-    </filter>
+        </include>
+    </coverage>
 </phpunit>
diff --git a/phpunit.xml.legacy b/phpunit.xml.legacy
new file mode 100644
index 0000000..fbb43e8
--- /dev/null
+++ b/phpunit.xml.legacy
@@ -0,0 +1,18 @@
+<?xml version="1.0" encoding="UTF-8"?>
+
+<!-- PHPUnit configuration file with old format for PHPUnit 9.2 or older -->
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/4.8/phpunit.xsd"
+         bootstrap="vendor/autoload.php"
+         colors="true">
+    <testsuites>
+        <testsuite name="React Test Suite">
+            <directory>./tests/</directory>
+        </testsuite>
+    </testsuites>
+    <filter>
+        <whitelist>
+            <directory>./src/</directory>
+        </whitelist>
+    </filter>
+</phpunit>
diff --git a/tests/TestCase.php b/tests/TestCase.php
index 43a68e4..48f2879 100644
--- a/tests/TestCase.php
+++ b/tests/TestCase.php
@@ -2,27 +2,49 @@
 
 namespace React\Tests\Async;
 
-class TestCase extends \PHPUnit_Framework_TestCase
+use PHPUnit\Framework\TestCase as BaseTestCase;
+
+class TestCase extends BaseTestCase
 {
-    protected function createCallableMock($expects, $with = null)
+    protected function expectCallableOnce()
     {
-        $callable = $this->getMockBuilder('React\Tests\Async\CallableStub')->getMock();
-
-        $method = $callable
-            ->expects($expects)
+        $mock = $this->createCallableMock();
+        $mock
+            ->expects($this->once())
             ->method('__invoke');
 
-        if ($with) {
-            $method->with($with);
-        }
+        return $mock;
+    }
 
-        return $callable;
+    protected function expectCallableOnceWith($value)
+    {
+        $mock = $this->createCallableMock();
+        $mock
+            ->expects($this->once())
+            ->method('__invoke')
+            ->with($value);
+
+        return $mock;
     }
-}
 
-class CallableStub
-{
-    public function __invoke()
+    protected function expectCallableNever()
     {
+        $mock = $this->createCallableMock();
+        $mock
+            ->expects($this->never())
+            ->method('__invoke');
+
+        return $mock;
+    }
+
+    protected function createCallableMock()
+    {
+        if (method_exists('PHPUnit\Framework\MockObject\MockBuilder', 'addMethods')) {
+            // PHPUnit 9+
+            return $this->getMockBuilder('stdClass')->addMethods(array('__invoke'))->getMock();
+        } else {
+            // legacy PHPUnit 4 - PHPUnit 8
+            return $this->getMockBuilder('stdClass')->setMethods(array('__invoke'))->getMock();
+        }
     }
 }
diff --git a/tests/UtilParallelTest.php b/tests/UtilParallelTest.php
index 022f78c..62c59b7 100644
--- a/tests/UtilParallelTest.php
+++ b/tests/UtilParallelTest.php
@@ -11,8 +11,8 @@ public function testParallelWithoutTasks()
     {
         $tasks = array();
 
-        $callback = $this->createCallableMock($this->once(), array());
-        $errback = $this->createCallableMock($this->never());
+        $callback = $this->expectCallableOnceWith(array());
+        $errback = $this->expectCallableNever();
 
         Util::parallel($tasks, $callback, $errback);
     }
@@ -32,8 +32,8 @@ function ($callback, $errback) {
             },
         );
 
-        $callback = $this->createCallableMock($this->once(), array('foo', 'bar'));
-        $errback = $this->createCallableMock($this->never());
+        $callback = $this->expectCallableOnceWith(array('foo', 'bar'));
+        $errback = $this->expectCallableNever();
 
         Util::parallel($tasks, $callback, $errback);
 
@@ -65,8 +65,8 @@ function ($callback, $errback) use (&$called) {
             },
         );
 
-        $callback = $this->createCallableMock($this->never());
-        $errback = $this->createCallableMock($this->once());
+        $callback = $this->expectCallableNever();
+        $errback = $this->expectCallableOnce();
 
         Util::parallel($tasks, $callback, $errback);
 
@@ -94,8 +94,8 @@ function ($callback, $errback) use (&$called) {
             },
         );
 
-        $callback = $this->createCallableMock($this->never());
-        $errback = $this->createCallableMock($this->once());
+        $callback = $this->expectCallableNever();
+        $errback = $this->expectCallableOnce();
 
         Util::parallel($tasks, $callback, $errback);
 
diff --git a/tests/UtilSeriesTest.php b/tests/UtilSeriesTest.php
index ff4ce53..95fa182 100644
--- a/tests/UtilSeriesTest.php
+++ b/tests/UtilSeriesTest.php
@@ -11,8 +11,8 @@ public function testSeriesWithoutTasks()
     {
         $tasks = array();
 
-        $callback = $this->createCallableMock($this->once(), array());
-        $errback = $this->createCallableMock($this->never());
+        $callback = $this->expectCallableOnceWith(array());
+        $errback = $this->expectCallableNever();
 
         Util::series($tasks, $callback, $errback);
     }
@@ -32,8 +32,8 @@ function ($callback, $errback) {
             },
         );
 
-        $callback = $this->createCallableMock($this->once(), array('foo', 'bar'));
-        $errback = $this->createCallableMock($this->never());
+        $callback = $this->expectCallableOnceWith(array('foo', 'bar'));
+        $errback = $this->expectCallableNever();
 
         Util::series($tasks, $callback, $errback);
 
@@ -65,8 +65,8 @@ function ($callback, $errback) use (&$called) {
             },
         );
 
-        $callback = $this->createCallableMock($this->never());
-        $errback = $this->createCallableMock($this->once());
+        $callback = $this->expectCallableNever();
+        $errback = $this->expectCallableOnce();
 
         Util::series($tasks, $callback, $errback);
 
diff --git a/tests/UtilWaterfallTest.php b/tests/UtilWaterfallTest.php
index e3b6052..b8d384b 100644
--- a/tests/UtilWaterfallTest.php
+++ b/tests/UtilWaterfallTest.php
@@ -11,8 +11,8 @@ public function testWaterfallWithoutTasks()
     {
         $tasks = array();
 
-        $callback = $this->createCallableMock($this->once(), array());
-        $errback = $this->createCallableMock($this->never());
+        $callback = $this->expectCallableOnce();
+        $errback = $this->expectCallableNever();
 
         Util::waterfall($tasks, $callback, $errback);
     }
@@ -37,8 +37,8 @@ function ($bar, $callback, $errback) {
             },
         );
 
-        $callback = $this->createCallableMock($this->once(), 'foobarbaz');
-        $errback = $this->createCallableMock($this->never());
+        $callback = $this->expectCallableOnceWith('foobarbaz');
+        $errback = $this->expectCallableNever();
 
         Util::waterfall($tasks, $callback, $errback);
 
@@ -70,8 +70,8 @@ function ($callback, $errback) use (&$called) {
             },
         );
 
-        $callback = $this->createCallableMock($this->never());
-        $errback = $this->createCallableMock($this->once());
+        $callback = $this->expectCallableNever();
+        $errback = $this->expectCallableOnce();
 
         Util::waterfall($tasks, $callback, $errback);