diff --git a/src/Directory.Packages.props b/src/Directory.Packages.props
index b60ae25..6c71bb8 100644
--- a/src/Directory.Packages.props
+++ b/src/Directory.Packages.props
@@ -21,13 +21,11 @@
   <ItemGroup>
     <PackageVersion Include="coverlet.collector" Version="6.0.4" />
     <PackageVersion Include="coverlet.msbuild" Version="6.0.4" />
-    <PackageVersion Include="FluentAssertions" Version="8.8.0" />
-    <PackageVersion Include="FluentAssertions.Analyzers" Version="0.34.1" />
-    <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="18.0.0" />
+    <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="18.0.1" />
     <PackageVersion Include="Microsoft.Reactive.Testing" Version="6.1.0" />
-    <PackageVersion Include="xunit" Version="2.9.3" />
-    <PackageVersion Include="xunit.analyzers" Version="1.25.0" />
-    <PackageVersion Include="xunit.runner.visualstudio" Version="3.1.5" />
+    <PackageVersion Include="NUnit" Version="4.4.0" />
+    <PackageVersion Include="NUnit3TestAdapter" Version="5.2.0" />
+    <PackageVersion Include="NUnit.Analyzers" Version="4.11.2" />
   </ItemGroup>
   
   <Import
diff --git a/src/ReactiveUI.Extensions.Tests/AssemblyInfo.Parallel.cs b/src/ReactiveUI.Extensions.Tests/AssemblyInfo.Parallel.cs
new file mode 100644
index 0000000..5614d58
--- /dev/null
+++ b/src/ReactiveUI.Extensions.Tests/AssemblyInfo.Parallel.cs
@@ -0,0 +1,8 @@
+// Copyright (c) 2019-2025 ReactiveUI Association Incorporated. All rights reserved.
+// ReactiveUI Association Incorporated licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for full license information.
+
+using NUnit.Framework;
+
+[assembly: Parallelizable(ParallelScope.Fixtures)]
+[assembly: LevelOfParallelism(4)]
diff --git a/src/ReactiveUI.Extensions.Tests/DisposableExtensionsTests.cs b/src/ReactiveUI.Extensions.Tests/DisposableExtensionsTests.cs
index 7ab185a..09ef9ce 100644
--- a/src/ReactiveUI.Extensions.Tests/DisposableExtensionsTests.cs
+++ b/src/ReactiveUI.Extensions.Tests/DisposableExtensionsTests.cs
@@ -5,8 +5,7 @@
 using System;
 using System.Reactive.Disposables;
 using System.Reactive.Disposables.Fluent;
-using FluentAssertions;
-using Xunit;
+using NUnit.Framework;
 
 namespace ReactiveUI.Extensions.Tests;
 
@@ -18,25 +17,23 @@ public class DisposableExtensionsTests
     /// <summary>
     /// Tests DisposeWith returns a disposable.
     /// </summary>
-    [Fact]
+    [Test]
     public void GivenNull_WhenDisposeWith_ThenExceptionThrown()
     {
         // Given
         var sut = Disposable.Create(() => { });
 
         // When
-        var result = Record.Exception(() => sut.DisposeWith(null!));
+        var result = Assert.Catch<Exception>(() => sut.DisposeWith(null!));
 
         // Then
-        result
-            .Should()
-            .BeOfType<ArgumentNullException>();
+        Assert.That(result, Is.TypeOf<ArgumentNullException>());
     }
 
     /// <summary>
     /// Tests DisposeWith disposes the underlying disposable.
     /// </summary>
-    [Fact]
+    [Test]
     public void GivenDisposable_WhenDisposeWith_ThenDisposed()
     {
         // Given
@@ -48,15 +45,13 @@ public void GivenDisposable_WhenDisposeWith_ThenDisposed()
         compositeDisposable.Dispose();
 
         // Then
-        sut.IsDisposed
-            .Should()
-            .BeTrue();
+        Assert.That(sut.IsDisposed, Is.True);
     }
 
     /// <summary>
     /// Tests DisposeWith returns the original disposable.
     /// </summary>
-    [Fact]
+    [Test]
     public void GivenDisposable_WhenDisposeWith_ThenReturnsDisposable()
     {
         // Given, When
@@ -65,7 +60,6 @@ public void GivenDisposable_WhenDisposeWith_ThenReturnsDisposable()
         var result = sut.DisposeWith(compositeDisposable);
 
         // Then
-        sut.Should()
-            .BeEquivalentTo(result);
+        Assert.That(result, Is.SameAs(sut));
     }
 }
diff --git a/src/ReactiveUI.Extensions.Tests/ReactiveExtensionsTests.cs b/src/ReactiveUI.Extensions.Tests/ReactiveExtensionsTests.cs
index 6914b8a..cf41aae 100644
--- a/src/ReactiveUI.Extensions.Tests/ReactiveExtensionsTests.cs
+++ b/src/ReactiveUI.Extensions.Tests/ReactiveExtensionsTests.cs
@@ -8,8 +8,7 @@
 using System.Reactive.Subjects;
 using System.Threading;
 using System.Threading.Tasks;
-using FluentAssertions;
-using Xunit;
+using NUnit.Framework;
 
 namespace ReactiveUI.Extensions.Tests;
 
@@ -21,7 +20,7 @@ public class ReactiveExtensionsTests
     /// <summary>
     /// Tests the WhereIsNotNull extension.
     /// </summary>
-    [Fact]
+    [Test]
     public void GivenNull_WhenWhereIsNotNull_ThenNoNotification()
     {
         // Given, When
@@ -29,15 +28,13 @@ public void GivenNull_WhenWhereIsNotNull_ThenNoNotification()
         using var disposable = Observable.Return<bool?>(null).WhereIsNotNull().Subscribe(x => result = x);
 
         // Then
-        result
-            .Should()
-            .BeNull();
+        Assert.That(result, Is.Null);
     }
 
     /// <summary>
     /// Tests the WhereIsNotNull extension.
     /// </summary>
-    [Fact]
+    [Test]
     public void GivenValue_WhenWhereIsNotNull_ThenNotification()
     {
         // Given, When
@@ -45,15 +42,13 @@ public void GivenValue_WhenWhereIsNotNull_ThenNotification()
         using var disposable = Observable.Return<bool?>(false).WhereIsNotNull().Subscribe(x => result = x);
 
         // Then
-        result
-            .Should()
-            .BeFalse();
+        Assert.That(result, Is.False);
     }
 
     /// <summary>
     /// Tests the AsSignal extension.
     /// </summary>
-    [Fact]
+    [Test]
     public void GivenObservable_WhenAsSignal_ThenNotifiesUnit()
     {
         // Given, When
@@ -61,15 +56,13 @@ public void GivenObservable_WhenAsSignal_ThenNotifiesUnit()
         using var disposable = Observable.Return<bool?>(false).AsSignal().Subscribe(x => result = x);
 
         // Then
-        result
-            .Should()
-            .Be(Unit.Default);
+        Assert.That(result, Is.EqualTo(Unit.Default));
     }
 
     /// <summary>
     /// Syncronizes the asynchronous runs with asynchronous tasks in subscriptions.
     /// </summary>
-    [Fact]
+    [Test]
     public void SubscribeSynchronus_RunsWithAsyncTasksInSubscriptions()
     {
         // Given, When
@@ -106,15 +99,13 @@ public void SubscribeSynchronus_RunsWithAsyncTasksInSubscriptions()
         }
 
         // Then
-        result
-            .Should()
-            .Be(0);
+        Assert.That(result, Is.Zero);
     }
 
     /// <summary>
     /// Syncronizes the asynchronous runs with asynchronous tasks in subscriptions.
     /// </summary>
-    [Fact]
+    [Test]
     public void SyncronizeAsync_RunsWithAsyncTasksInSubscriptions()
     {
         // Given, When
@@ -153,15 +144,13 @@ public void SyncronizeAsync_RunsWithAsyncTasksInSubscriptions()
         }
 
         // Then
-        result
-            .Should()
-            .Be(0);
+        Assert.That(result, Is.Zero);
     }
 
     /// <summary>
     /// Syncronizes the asynchronous runs with asynchronous tasks in subscriptions.
     /// </summary>
-    [Fact]
+    [Test]
     public void SynchronizeSynchronous_RunsWithAsyncTasksInSubscriptions()
     {
         // Given, When
@@ -200,15 +189,13 @@ public void SynchronizeSynchronous_RunsWithAsyncTasksInSubscriptions()
         }
 
         // Then
-        result
-            .Should()
-            .Be(0);
+        Assert.That(result, Is.Zero);
     }
 
     /// <summary>
     /// Syncronizes the asynchronous runs with asynchronous tasks in subscriptions.
     /// </summary>
-    [Fact]
+    [Test]
     public void SubscribeAsync_RunsWithAsyncTasksInSubscriptions()
     {
         // Given, When
@@ -245,8 +232,6 @@ public void SubscribeAsync_RunsWithAsyncTasksInSubscriptions()
         }
 
         // Then
-        result
-            .Should()
-            .Be(0);
+        Assert.That(result, Is.Zero);
     }
 }
diff --git a/src/ReactiveUI.Extensions.Tests/ReactiveUI.Extensions.Tests.csproj b/src/ReactiveUI.Extensions.Tests/ReactiveUI.Extensions.Tests.csproj
index 9f240da..fdc581d 100644
--- a/src/ReactiveUI.Extensions.Tests/ReactiveUI.Extensions.Tests.csproj
+++ b/src/ReactiveUI.Extensions.Tests/ReactiveUI.Extensions.Tests.csproj
@@ -6,18 +6,13 @@
     </PropertyGroup>
   
   <ItemGroup>
-    <PackageReference Include="FluentAssertions" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" />
     <PackageReference Include="Microsoft.Reactive.Testing" />
-    <PackageReference Include="xunit" />
-    <PackageReference Include="xunit.runner.visualstudio">
-      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
-      <PrivateAssets>all</PrivateAssets>
-    </PackageReference>
-    <PackageReference Include="coverlet.collector">
-      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
-      <PrivateAssets>all</PrivateAssets>
-    </PackageReference>
+    <PackageReference Include="NUnit" />
+    <PackageReference Include="NUnit3TestAdapter" />
+    <PackageReference Include="coverlet.collector" PrivateAssets="all" />
+    <PackageReference Include="coverlet.msbuild"  />
+    <PackageReference Include="NUnit.Analyzers" PrivateAssets="all" />
   </ItemGroup>
   
   <ItemGroup>
diff --git a/tests.runsettings b/tests.runsettings
new file mode 100644
index 0000000..e5d4ab7
--- /dev/null
+++ b/tests.runsettings
@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="utf-8"?>
+<RunSettings>
+  <NUnit>
+    <NumberOfTestWorkers>4</NumberOfTestWorkers>
+  </NUnit>
+</RunSettings>