diff --git a/composer.json b/composer.json
index 67f9c628..71432283 100644
--- a/composer.json
+++ b/composer.json
@@ -15,7 +15,7 @@
         }
     },
     "require": {
-        "php": "^7.1",
+        "php": "^7.1 || ^8.0",
         "composer/package-versions-deprecated": "1.11.99.2",
         "elife/api-client": "^1.0",
         "guzzlehttp/promises": "^1.0",
@@ -32,7 +32,7 @@
         "mtdowling/jmespath.php": "^2.0",
         "phpspec/phpspec": "^5.1 || ^6.3 || ^7.5",
         "phpspec/prophecy": "^1.10",
-        "phpunit/phpunit": "^7.5",
+        "phpunit/phpunit": "^7.5 || ^9.6",
         "psr/http-message": "^1.0",
         "squizlabs/php_codesniffer": "^3.5"
     },
diff --git a/src/ArrayFromIterator.php b/src/ArrayFromIterator.php
index cdad00ad..ba47b6c3 100644
--- a/src/ArrayFromIterator.php
+++ b/src/ArrayFromIterator.php
@@ -65,7 +65,7 @@ final public function toArray() : array
         return $array;
     }
 
-    final private function all() : PromiseSequence
+    private function all() : PromiseSequence
     {
         return new PromiseSequence(
             $promise = new Promise(
diff --git a/src/CanBeSliced.php b/src/CanBeSliced.php
index 621ec671..95f7f5a0 100644
--- a/src/CanBeSliced.php
+++ b/src/CanBeSliced.php
@@ -14,7 +14,7 @@ trait CanBeSliced
 
     abstract public function slice(int $offset, int $length = null) : Sequence;
 
-    final private function getPage(int $page) : Sequence
+    private function getPage(int $page) : Sequence
     {
         if (empty($this->pages)) {
             for ($i = 0; $i < $this->count(); ++$i) {
@@ -31,7 +31,7 @@ final private function getPage(int $page) : Sequence
         return $this->pages[$page];
     }
 
-    final private function resetPages()
+    private function resetPages()
     {
         $this->pages = [];
         $this->pageBatch = 100;
diff --git a/src/Collection/ArraySequence.php b/src/Collection/ArraySequence.php
index 3fecfce2..314cb98e 100644
--- a/src/Collection/ArraySequence.php
+++ b/src/Collection/ArraySequence.php
@@ -141,6 +141,7 @@ public function offsetExists($offset) : bool
         return isset($this->array[$offset]);
     }
 
+    #[\ReturnTypeWillChange]
     public function offsetGet($offset)
     {
         if (!isset($this->array[$offset])) {
diff --git a/src/Collection/EmptySequence.php b/src/Collection/EmptySequence.php
index 278e6811..4a3a555a 100644
--- a/src/Collection/EmptySequence.php
+++ b/src/Collection/EmptySequence.php
@@ -94,6 +94,7 @@ public function offsetExists($offset) : bool
         return false;
     }
 
+    #[\ReturnTypeWillChange]
     public function offsetGet($offset)
     {
         return null;
diff --git a/src/Collection/PromiseSequence.php b/src/Collection/PromiseSequence.php
index b57e52b0..a996feda 100644
--- a/src/Collection/PromiseSequence.php
+++ b/src/Collection/PromiseSequence.php
@@ -191,6 +191,7 @@ public function offsetExists($offset) : bool
         return $this->wait()->offsetExists($offset);
     }
 
+    #[\ReturnTypeWillChange]
     public function offsetGet($offset)
     {
         return $this->wait()->offsetGet($offset);
diff --git a/src/ImmutableArrayAccess.php b/src/ImmutableArrayAccess.php
index c7043e1e..e8d42440 100644
--- a/src/ImmutableArrayAccess.php
+++ b/src/ImmutableArrayAccess.php
@@ -6,12 +6,12 @@
 
 trait ImmutableArrayAccess
 {
-    final public function offsetSet($offset, $value)
+    final public function offsetSet($offset, $value): void
     {
         throw new BadMethodCallException('Object is immutable');
     }
 
-    final public function offsetUnset($offset)
+    final public function offsetUnset($offset): void
     {
         throw new BadMethodCallException('Object is immutable');
     }
diff --git a/src/Model/SearchSubjects.php b/src/Model/SearchSubjects.php
index a7e6367e..60d765b4 100644
--- a/src/Model/SearchSubjects.php
+++ b/src/Model/SearchSubjects.php
@@ -19,33 +19,35 @@ public function __construct(array $subjects, array $results)
         $this->results = $results;
     }
 
-    public function count()
+    public function count(): int
     {
         return count($this->subjects);
     }
 
+    #[\ReturnTypeWillChange]
     public function current()
     {
         return current($this->results);
     }
 
-    public function next()
+    public function next(): void
     {
         next($this->subjects);
         next($this->results);
     }
 
+    #[\ReturnTypeWillChange]
     public function key()
     {
         return current($this->subjects);
     }
 
-    public function valid()
+    public function valid(): bool
     {
         return false !== $this->key();
     }
 
-    public function rewind()
+    public function rewind(): void
     {
         reset($this->subjects);
         reset($this->results);
diff --git a/src/Model/SearchTypes.php b/src/Model/SearchTypes.php
index 0f9672eb..b600dadc 100644
--- a/src/Model/SearchTypes.php
+++ b/src/Model/SearchTypes.php
@@ -19,7 +19,7 @@ public function __construct(array $typesToResults)
         $this->typesToResults = $typesToResults;
     }
 
-    public function count()
+    public function count(): int
     {
         return count($this->typesToResults);
     }
diff --git a/src/SlicedArrayAccess.php b/src/SlicedArrayAccess.php
index e19c37fb..35d8c3e0 100644
--- a/src/SlicedArrayAccess.php
+++ b/src/SlicedArrayAccess.php
@@ -14,6 +14,7 @@ public function offsetExists($offset) : bool
         return null !== $this->offsetGet($offset);
     }
 
+    #[\ReturnTypeWillChange]
     public function offsetGet($offset)
     {
         try {
diff --git a/src/SlicedIterator.php b/src/SlicedIterator.php
index 6aae7299..18ac3bca 100644
--- a/src/SlicedIterator.php
+++ b/src/SlicedIterator.php
@@ -10,6 +10,7 @@ trait SlicedIterator
 
     private $key = 1;
 
+    #[\ReturnTypeWillChange]
     final public function current()
     {
         $page = (int) ceil($this->key / $this->pageBatch);
@@ -23,11 +24,12 @@ final public function current()
         return $pageContents[$inPage];
     }
 
-    final public function next()
+    final public function next(): void
     {
         ++$this->key;
     }
 
+    #[\ReturnTypeWillChange]
     final public function key()
     {
         if ($this->key > $this->count()) {
@@ -37,17 +39,17 @@ final public function key()
         return $this->key;
     }
 
-    final public function valid()
+    final public function valid(): bool
     {
         return $this->key <= $this->count();
     }
 
-    final public function rewind()
+    final public function rewind(): void
     {
         $this->key = 1;
     }
 
-    final private function resetIterator()
+    private function resetIterator()
     {
         $this->rewind();
         $this->resetPages();
diff --git a/test/ApiSdkTest.php b/test/ApiSdkTest.php
index 68f04320..a2873e25 100644
--- a/test/ApiSdkTest.php
+++ b/test/ApiSdkTest.php
@@ -133,8 +133,7 @@ public function it_creates_community()
         $this->mockCommunityListCall(1, 1, 1);
         $this->mockBlogArticleCall('model-1', true);
 
-        $this->assertInternalType(
-            'array',
+        $this->assertIsArray(
             $this->apiSdk->getSerializer()->normalize($this->apiSdk->community()[0])
         );
     }
diff --git a/test/Client/DigestsTest.php b/test/Client/DigestsTest.php
index 2f4e6d43..59615d39 100644
--- a/test/Client/DigestsTest.php
+++ b/test/Client/DigestsTest.php
@@ -126,7 +126,7 @@ public function it_gets_a_digest()
         $this->assertSame('Digest digest-7 text', $digest->getContent()[0]->getText());
 
         $this->assertInstanceOf(ArticleVoR::class, $digest->getRelatedContent()[0]);
-        $this->assertContains('Homo naledi', $digest->getRelatedContent()[0]->getTitle());
+        $this->assertStringContainsString('Homo naledi', $digest->getRelatedContent()[0]->getTitle());
 
         $this->mockArticleCall('09560', true, true, 1);
 
diff --git a/test/Client/SearchTest.php b/test/Client/SearchTest.php
index 07a50dd5..b3724745 100644
--- a/test/Client/SearchTest.php
+++ b/test/Client/SearchTest.php
@@ -412,7 +412,7 @@ public function it_has_counters_for_types()
 
         $types = $this->search->types();
         foreach ($types as $type => $counter) {
-            $this->assertInternalType('string', $type);
+            $this->assertIsString($type);
             $this->assertRegexp('/^[a-z-]+$/', $type);
             $this->assertGreaterThanOrEqual(0, $counter);
         }
diff --git a/test/Model/ArticlePoATest.php b/test/Model/ArticlePoATest.php
index 80e5ec86..d6964a13 100644
--- a/test/Model/ArticlePoATest.php
+++ b/test/Model/ArticlePoATest.php
@@ -7,7 +7,7 @@
 
 final class ArticlePoATest extends ArticleVersionTest
 {
-    public function setUp()
+    public function setUp(): void
     {
         $this->builder = Builder::for(ArticlePoA::class);
     }
diff --git a/test/Model/ArticleVoRTest.php b/test/Model/ArticleVoRTest.php
index 801e2a8d..5c0fbad8 100644
--- a/test/Model/ArticleVoRTest.php
+++ b/test/Model/ArticleVoRTest.php
@@ -25,7 +25,7 @@
 
 final class ArticleVoRTest extends ArticleVersionTest
 {
-    public function setUp()
+    public function setUp(): void
     {
         $this->builder = Builder::for(ArticleVoR::class);
     }
diff --git a/test/Model/CollectionTest.php b/test/Model/CollectionTest.php
index ffc4353b..e14857a6 100644
--- a/test/Model/CollectionTest.php
+++ b/test/Model/CollectionTest.php
@@ -32,7 +32,7 @@ final class CollectionTest extends TestCase
 {
     private $builder;
 
-    public function setUp()
+    public function setUp(): void
     {
         $this->builder = Builder::for(Collection::class);
     }
diff --git a/test/Model/DigestTest.php b/test/Model/DigestTest.php
index 019fce2a..fb532103 100644
--- a/test/Model/DigestTest.php
+++ b/test/Model/DigestTest.php
@@ -30,7 +30,7 @@ final class DigestTest extends TestCase
 {
     private $builder;
 
-    public function setUp()
+    public function setUp(): void
     {
         $this->builder = Builder::for(Digest::class);
     }
diff --git a/test/Model/IdentifierTest.php b/test/Model/IdentifierTest.php
index a27821cf..5597271f 100644
--- a/test/Model/IdentifierTest.php
+++ b/test/Model/IdentifierTest.php
@@ -13,7 +13,7 @@ final class IdentifierTest extends TestCase
      * @test
      * @dataProvider identifierProvider
      */
-    public function it_has_a_type(string $method, string $type, string $id)
+    public function it_has_a_type(string $method, string $type, $id)
     {
         $identifier = Identifier::{$method}($id);
 
@@ -24,7 +24,7 @@ public function it_has_a_type(string $method, string $type, string $id)
      * @test
      * @dataProvider identifierProvider
      */
-    public function it_has_an_id(string $method, string $type, string $id)
+    public function it_has_an_id(string $method, string $type, $id)
     {
         $identifier = Identifier::{$method}($id);
 
@@ -35,7 +35,7 @@ public function it_has_an_id(string $method, string $type, string $id)
      * @test
      * @dataProvider identifierProvider
      */
-    public function it_can_be_created_from_a_string(string $method, string $type, string $id)
+    public function it_can_be_created_from_a_string(string $method, string $type, $id)
     {
         $identifier = Identifier::fromString("$type/$id");
 
diff --git a/test/Model/PressPackageTest.php b/test/Model/PressPackageTest.php
index 57a5a421..ee5ee2c1 100644
--- a/test/Model/PressPackageTest.php
+++ b/test/Model/PressPackageTest.php
@@ -28,7 +28,7 @@ final class PressPackageTest extends TestCase
 {
     private $builder;
 
-    public function setUp()
+    public function setUp(): void
     {
         $this->builder = Builder::for(PressPackage::class);
     }
diff --git a/test/Model/PromotionalCollectionTest.php b/test/Model/PromotionalCollectionTest.php
index 4204169c..b08ea291 100644
--- a/test/Model/PromotionalCollectionTest.php
+++ b/test/Model/PromotionalCollectionTest.php
@@ -32,7 +32,7 @@ final class PromotionalCollectionTest extends TestCase
 {
     private $builder;
 
-    public function setUp()
+    public function setUp(): void
     {
         $this->builder = Builder::for(PromotionalCollection::class);
     }
diff --git a/test/Model/ReviewedPreprintTest.php b/test/Model/ReviewedPreprintTest.php
index 0f126374..67f400e9 100644
--- a/test/Model/ReviewedPreprintTest.php
+++ b/test/Model/ReviewedPreprintTest.php
@@ -25,7 +25,7 @@ final class ReviewedPreprintTest extends TestCase
 {
     private $builder;
 
-    public function setUp()
+    public function setUp(): void
     {
         $this->builder = Builder::for(ReviewedPreprint::class);
     }
diff --git a/test/Model/SearchSubjectsTest.php b/test/Model/SearchSubjectsTest.php
index 4466d48b..4fffde49 100644
--- a/test/Model/SearchSubjectsTest.php
+++ b/test/Model/SearchSubjectsTest.php
@@ -27,7 +27,7 @@ public function it_is_iterable_with_subjects_as_keys()
         foreach ($searchSubjects as $subject => $results) {
             $this->assertInstanceOf(Subject::class, $subject);
             $actualSubjects[] = $subject;
-            $this->assertInternalType('integer', $results);
+            $this->assertIsInt($results);
             $actualResults[] = $results;
         }
 
diff --git a/test/TestCase.php b/test/TestCase.php
index 76778cb0..b608014b 100644
--- a/test/TestCase.php
+++ b/test/TestCase.php
@@ -29,8 +29,8 @@ final protected function classNameProvider(string ...$classes) : Traversable
 
     final protected function assertObjectsAreEqual($expected, $actual, $detail = '')
     {
-        $this->assertInternalType('object', $expected, $detail);
-        $this->assertInternalType('object', $actual, $detail);
+        $this->assertIsObject($expected, $detail);
+        $this->assertIsObject($actual, $detail);
         $this->assertInstanceOf(get_class($expected), $actual, $detail);
 
         foreach (get_class_methods($actual) as $method) {
@@ -59,7 +59,7 @@ private function assertItemsAreEqual($expected, $actual, $detail = null)
         if (is_object($actual)) {
             $this->assertObjectsAreEqual($expected, $actual, $detail);
         } elseif (is_array($actual)) {
-            $this->assertInternalType('array', $expected, "We are getting an array out of $detail but we were not expecting it");
+            $this->assertIsArray($expected, "We are getting an array out of $detail but we were not expecting it");
             $this->assertEquals(count($expected), count($actual), "Count of $detail doesn't match expected");
             foreach ($actual as $key => $actualItem) {
                 $this->assertItemsAreEqual($expected[$key], $actualItem, $detail.' '.$key);