Where to find "Microsoft.VisualStudio.TestTools.UnitTesting" missing dll?

Question

You have to add reference to

Microsoft.VisualStudio.QualityTools.UnitTestFramework.dll 

It can be found at C:\Program Files\Microsoft Visual Studio 10.0\Common7\IDE\PublicAssemblies\ directory (for VS2010 professional or above; .NET Framework 4.0).

or right click on your project and select: Add Reference... > .NET:

I know this is old, this is what came up in my Google search. I needed to reference these packages on NuGet:

The DLL you're looking for that contains that namespace is

Microsoft.VisualStudio.QualityTools.UnitTestFramework.dll

Note that unit testing cannot be used in Visual Studio Express.

To resolve this issue, I had to do the following:

1. Launch the Visual Studio Installer with administrative privileges
2. If it prompts you to install updates to Visual Studio, do so before continuing
3. When prompted, click the button to Modify the existing installation
4. Click on the "Individual components" tab / header along the top
5. Scroll down to the "Debugging and testing" section
6. Check the box next to "Web performance and load testing tools"
7. Click the Modify button on the bottom right corner of the dialog to install the missing DLLs

Once the DLLs are installed, you can add references to them using the method that Agent007 indicated in his answer.

There is also a nice nuget package. It will pull the dll to your packages folder. You will need to add the reference to the dll manually.

NOTE: This package is not an official Microsoft package.

If you are using Visual Studio 2017 Community, the location is:

C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\Common7\IDE\ReferenceAssemblies\v2.0

The DLL you want is there: Microsoft.VisualStudio.QualityTools.UnitTestFramework.dll

Apparently it is located in the C:\Program Files\Microsoft Visual Studio 10.0\Common7\IDE\PublicAssemblies\ directory for Visual Studio 2010 Professional version, but take note that the 10.0 will change to correspond with the release year, i.e. VS 2013 was version 12.0, VS 2015 was version 14.0, VS 2017 is 15.0. (VS Express is not supported and would require installing the NUnit NuGet package, through the NuGet Package Manager, instead.)

You go to References, right-click, select Add Reference, Browse. Navigate to the path, then double-click the file.

Then, you need a using statement at the top of your Unit Test class:

using Microsoft.VisualStudio.TestTools.UnitTesting;