Reliable Broadcast is a mechanism by which a processor in a distributed system disseminates a value to all other processors in the presence of both communication and processor failures. Protocols to achieve Reliable Broadcast are at the heart of most fault-tolerant applications. We characterize the execution time of Reliable Broadcast protocols as a function of the communication model. This model includes familiar communication structures such as fully-connected point-to-point graphs, linear chains, rings, broadcast networks (such as Ethernet) and buses. We derive a parameterized protocol that implements Reliable Broadcast for any member within this class. We obtain lower bound results that show the optimality of our protocols. The lower bound results identify a time complexity gap between systems where processors may only fail to send messages, and systems where processors may fail both to send and to receive messages. The tradeoffs that our results reveal between performance, resiliency and network cost offer many new alternatives previously not considered in designing fault-tolerant systems. © 1988 Springer-Verlag.24177189