Abstract Colin Campbell, Wolfgang Grieskamp, Lev Nachmanson, Wolfram Schulte, Nikolai Tillmann, and Margus Veanes Microsoft Research, Redmond, WA, USA Abstract. We describe a practical model-based testing tool developed at Microsoft Research called Spec Explorer. Spec Explorer enables modeling and automatic testing of concurrent object-oriented systems. These systems take inputs as well as provide outputs in form of spontaneous reactions, where inputs and outputs can be arbitrary data types, including objects. Spec Explorer is being used daily by several Microsoft product groups. The here presented techniques are used to test operating system components and Web service infrastructure. Transition Systems Formalize Reactive and Distributed Systems. Reactive and distributed systems are inherently nondeterministic. No single agent (component, thread, etc.) controls all state transitions, and even external entities like the operating systems scheduler or the network may play a role. A pract...