Version 1 of NUnit used the classic approach to identifying tests based on inheritance and naming conventions. From version 2.0 on, NUnit has used custom attributes for this purpose.
Because NUnit test fixtures do not inherit from a framework class, the developer is free to use inheritance in other ways. And because there is no arbitrary convention for naming tests, the choice of names can be entirely oriented toward communicating the purpose of the test.
All NUnit attributes are contained in the NUnit.Framework namespace. Each source file that contains tests must include a using statement for that namespace and the project must reference the framework assembly, nunit.framework.dll.