Many tools and techniques have been developed to address specific aspects of interacting in a virtual world. Few have been designed with an architecture that allows large numbers of entities from disparate organizations to interact in such a world, in real time, and over large geographic distances. A system architecture that does this is described. The key technologies that have made these virtual worlds possible are discussed, and it is explained and how the technologies fit into the architecture. A sample implementation of this architecture, the SIMulation NETworking (SIMNET) system, is then presented, along with various design decisions and the reasoning behind them.<<ETX>>