The article presents an approach to model based testing of complex systems based on a generalization of finite state machines (FSM) and input output state machines (IOSM). The approach presented is used in the context of UniTesK specification based test development method. The results of its practical applications are also discussed. Practical experience demonstrates the applicability of the approach for model based testing of protocol implementations, distributed and concurrent systems, and real-time systems. This work stems from ISPRAS results of academic research and industrial application of formal techniques in verification and testing [1].