Witnessing subsystems for probabilistic reachability thresholds in discrete Markovian models are an important concept both as diagnostic information on why a property holds, and as input to refinement algorithms. We present SWITSS, a tool for the computation of Small WITnessing SubSystems. SWITSS implements exact and heuristic approaches based on reducing the problem to (mixed integer) linear programming. Returned subsystems can automatically be rendered graphically and are accompanied with a certificate which proves that the subsystem is indeed a witness.