Replicated execution of distributed programs, which provides a means of masking hardware (processor) failures in a distributed system, is discussed. Application-level entities (processes, objects) are replicated to execute on distinct processors. Such replica entities communicate by message passing. Nondeterminism within the replicas could cause messages to be processed in nonidentical order, producing a divergence of state. Possible sources of nondeterminism are identified, and a generic mechanism for ensuring that nonfaulty replicas process messages in identical order, thereby preventing state divergence among such replicate entities, is presented.< >