The documentation for throwTo says: throwTo does not return until the exception has been raised in the target thread. The calling thread can thus be certain that the target thread has received the exception. This is a useful property to know when dealing with race conditions: eg. if there are two threads that can kill each other, it is guaranteed that only one of the threads will get to kill the other. I don't see how the last sentense follows. I understood it so that the implication throwTo has returned => exception has been delivered is true, but not the reverse. If my understanding is correct, then both exceptions could be delivered without any of throwTos returning. Or is it true that throwTo has returned <=> exception has been delivered ? Roman