This paper describes a new way of testing reactive systems as investigated by the RATE-project at the Tampere University of Technology. We abandon the idea of systematically using a large library of predetermined test cases and instead use a "live" specification to generate test runs on- the-fly, as testing progresses. In order to do this, we assume that the behavior of the implementation under test is specified as a labelled transition system. This testing method is most applicable to testing concurrent, nondeterministic, and reactive behaviors rather than data-intensive computation.