This thesis describes the development of a distributed computer architecture that supports the interconnection of loosely-coupled computing resources (sites) by heterogeneous communication networks. The internetwork system, named the intelligent message transport system, provides for reliable delivery of intersite messages even though the interconnecting networks may use a probabilistic ("best-effort") delivery scheme. Processors are interfaced to each other by a network gateway processor. The gateway processors are autonomous network controllers that provide a simple and consistent site interface to the internetwork system. The gateway processors also provide for high level functions at the Transport Layer of the system rather than burdening the user with networking details at the Application Layer. These high-level functions include the synchronization of concurrent updates to replicated data objects, and the management of the system-wide name space for shared data.