Everyone, I did just get around to give the MSM a first shot and I have say I am impressed at the neat design, the ease of use, "understandability", tutorial ... exceptional work. Really great and it helped me a great deal so far. So thanks a bunch for this teriffic addition to boost. That's the kind of stuff I love boost for. There is one question however that puzzles me. How can / do I prevent state transistions when a transition function throws? Most transition functions are defined like that: void so_transition(event::Cause const &e) { methodCausingAnException(); } The tutorial only mentions this at the very end of the back-end docs and this is not clear to me. I have stuff inside my transitions that may throw and the example text mentiones other valid examples (like connect() logic). So what am I supposed to do here? This? void so_transition(event::Cause const &e) { try { methodCausingAnException(); } catch (const std::exception &sex) { return ??? } } The type is void and I can't return anything that would prevent the transition. Or should I just let the exception pass? What is the correct way? Suggestions are appreciated, Stephan