In multiprogramming systems, parallel processes compete for access to shared resources and cooperate by exchanging information. Semaphores are a useful means for controlling competition and synchronizing execution and inter-process messages are useful for communication. Neither semaphores nor inter-process message, however, are natural for solving both problems. This paper introduces a new approach, message classes, which combines and extends features of both semaphores and message passing. Using message classes, numerous mutual exclusion, producer/consumer, process communication, and resource allocation problems can be readily solved.