IIRC, windows thread ids can be re-used. That is, if you stop a thread X and later start a thread Y they might get the same thread id. Is this the case with boost.thread? What about the standard? It states: "An object of type thread::id provides a unique identifier for each thread of execution and a single distinct value for all thread objects that do not represent a thread of execution ([thread.threads.class]). Each thread of execution has a thread::id that is not equal to the thread::id of other threads of execution and that is not equal to the thread::id of std::thread objects that do not represent threads of execution." http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2497.html#thread.th... Which sounds like thread ids are free to be re-used. Johan -- View this message in context: http://www.nabble.com/-thread--Are-thread-ids-unique-during-an-applications-... Sent from the Boost - Dev mailing list archive at Nabble.com.