Much of the current work in distributed, object-oriented systems is based on the assumption that objects form a single ontological class. This class consists of all entities that can be fully described by the specification of the set of interfaces supported by the object and the semantics of the operations in those interfaces. The class includes objects that share a single address space, objects that are in separate address spaces on the same machine, and objects that are in separate address spaces on different machines (with, perhaps, different architectures). On the view that all objects are essentially the same kind of entity, these differ- ences in relative location are merely an aspect of the implementation of the object. Indeed, the location of an object may change over time, as an object migrates from one machine to another or the implementation of the object changes.