An important component of the Mach design is the use of memory objects which can be managed either by the kernel or by user programs through a message interface. This feature allows applications such as transaction management systems to participate in decisions regarding secondary storage management and page replacement. This paper explores the goals, design and implementation of Mach and its external memory management facility. The relationship between memory and communication in Mach is examined as it relates to overall performance, applicability of Mach to new multiprocessor architectures, and the structure of application programs.