Hi, I tried to make it work, but sadly I didn't succeed. It almost looks like the PseudoExit state doesn't forward the event to the TopState. I currently commented out all other states in the SubState to make debugging easier and set the last State (Transport) to the initial_state. Of course I left Exit1 and Exit2 as they were. As i keep triggering Sensor Events ( they also get handled later on in the state machine ), it's almost like it fights its way through the errors... I have put the timeline with the code, debug prints and my comments below: row< Transport , Sensor , Exit2 , &SubFront::ActionSensor , &SubFront::SDownstreamNoCheck > Current state: 0 Event: Sensor SubState Guard: Sensor Downstream Active and No Check? SubState Action: Sensor SubState no_transition: Invalid state transition! --> 2 Type: N10controller19TopStateFront6SensorE, N5boost3msm4back13state_machineIN10controller19TopStateFront6SubStateAFrontENS_9parameter5void_ES8_S8_S8_EE The Current state in the debug prints is for SubState. I don't know which states these exactly are, but I guess Transport is state 0 and Exit2 is state 2. So, you see that the first sensor action triggers the transition and moves from Transport to Exit2. Why is no_transition called when the event already finished? Current state: 2 Event: Sensor SubState no_transition: Invalid state transition! --> 2 Type: N10controller19TopStateFront6SensorE, N5boost3msm4back13state_machineIN10controller19TopStateFront6SubStateAFrontENS_9parameter5void_ES8_S8_S8_EE TopState Guard: Exit2 TopState Action: Sensor After sending another Sensor event the SubState still complains about no_transition. Because I have defined guards for all transitions with debug prints, the failing transition is most likely generated from within the PseudoExit. As you can see it does execute the Exit2 guard and completes the transition. This code in the TopState will handle it: row< Sub::exit_pt< SubFront::Exit2 > , Sensor , SituationB , &TopFront::ActionSensor , &TopFront::Exit2Guard >, And Exit2 is defined (in SubState) as: struct Exit2 : public msm::front::exit_pseudo_state<Sensor> {}; Current state: 2 Event: Sensor TopState Guard: Situation B and beyond TopState Action: Sensor Here we are, in Situation B. So, one way or another ... it actually works. Strange... Any ideas?? Best regards, Johan