The xAMp is a highly versatile group communications service aimed at supporting the development of distributed applications with different dependability, functionality, and performance requirements. These range from unreliable and nonordered to atomic multicast, and are enhanced by efficient group addressing and management support. The basic protocols are synchronous, clockless and designed to be used over broadcast local area networks (LANs), and are portable to a number of them. The functionality provided yields a reasonably complete solution to the problem of reliable group communication. While other protocols offer similar services, the authors follow a novel engineering approach by deriving all qualities of services from a single basic procedure. Thus, their implementation shares data structures, procedures, failure-recovery algorithms, and group monitor services, resulting in a highly integrated package.<>