Hi Andreas, Many thanks for your help first of all! Upon your request, I wrote a small but complete program that demonstrates the issue. This program consists of 5 source code files: 1) main.cpp 2) StateMachine.hpp 3) StateMachine.cpp 4) SuperState.hpp 5) SubState.hpp The contents of these files are listed at the end of this e-mail. You should be able to compile these listings and then run the executable successfully. Depending on your compiler flags, you may receive one warning that says "/* within comment". This is intentional, so please ignore the warning. To reproduce my problem regarding the nesting of states, please modify the listings in three locations, as explained in the comments in SuperState.hpp and SubState.hpp, and then re-compile. You should receive multiple errors pointing to the Boost Statechart library. Looking forward to your feedback. Thanks & Regards, Gurcan ---------------------------------------- 1) main.cpp ---------------------------------------- #include <iostream> #include <boost/bind.hpp> #include <boost/function.hpp> #include <boost/thread/thread.hpp> #include <boost/statechart/asynchronous_state_machine.hpp> #include <boost/statechart/fifo_worker.hpp> #include "StateMachine.hpp" int main() { std::cout<<"\n\nPress any key and then <CR> to exit\n\n"<<std::endl; char key; boost::statechart::fifo_scheduler<> scheduler(true); boost::statechart::fifo_scheduler<>::processor_handle handle = scheduler.create_processor<StateMachine> (); scheduler.initiate_processor(handle); boost::thread aThread(boost::bind(&boost::statechart::fifo_scheduler<>::operator(),&schedu ler, 0)); std::cin >> key; scheduler.destroy_processor(handle); scheduler.terminate(); aThread.join(); return 0; } ---------------------------------------- 2) StateMachine.hpp ---------------------------------------- #ifndef StateMachine_hpp #define StateMachine_hpp #include <boost/statechart/asynchronous_state_machine.hpp> class StateMachine; class SuperState; namespace boost { namespace statechart { template<> inline void asynchronous_state_machine<StateMachine, SuperState>::initiate_impl() {} }} class StateMachine: public boost::statechart::asynchronous_state_machine<StateMachine, SuperState> { public: StateMachine(my_context ctx) : my_base(ctx) {} private: virtual void initiate_impl(); }; #endif ---------------------------------------- 3) StateMachine.cpp ---------------------------------------- #include <boost/statechart/state_machine.hpp> #include "StateMachine.hpp" #include "SuperState.hpp" void StateMachine::initiate_impl() { boost::statechart::state_machine<StateMachine, SuperState>::initiate(); } ---------------------------------------- 4) SuperState.hpp ---------------------------------------- #ifndef SuperState_hpp #define SuperState_hpp #include <iostream> #include <boost/statechart/simple_state.hpp> #include "StateMachine.hpp" #include "SubState.hpp" class StateMachine; class SubState; //MODIFICATION 1 of 3: UNCOMMENT THE FOLLOWING LINE //class SuperState: public boost::statechart::simple_state<SuperState, StateMachine, SubState> { //MODIFICATION 2 of 3: COMMENT OUT THE FOLLOWING LINE class SuperState: public boost::statechart::simple_state<SuperState, StateMachine> { public: SuperState(){ std::cout<<"Entered into SuperState."<<std::endl; } }; #endif ---------------------------------------- 5) SubState.hpp ---------------------------------------- //MODIFICATION 3 of 3: COMMENT OUT THE FOLLOWING LINE /* #ifndef SubState_hpp #define SubState_hpp #include <iostream> #include <boost/statechart/simple_state.hpp> #include "SuperState.hpp" class SuperState; class SubState: public boost::statechart::simple_state<SubState, SuperState> { public: SubState(){ std::cout<<"Enteres into SubState."<<std::endl; } }; #endif /**/