The first phase of virtual world development has focused on the novel hardware (3-D input and 3-D output) and the graphics demo. The second phase of virtual worlds development will be to focus in on the more significant part of the problem, the software bed underlying “real” applications. The focus of this paper is on the software required to support large scale, networked, multiparty virtual environments. We discuss navigation (virtual camera view point control and its coupling to real-time, hidden surface elimination), interaction (software for constructing a dialogue from the inputs read from our devices and for applying that dialogue to display changes), communication (software for passing changes in the world model to other players on the network, and software for allowing the entry of previously undescribed players into the system), autonomy (software for playing autonomous agents in our virtual world against interactive players), scripting (software for recording, playing back, and multitracking previous play against live or autonomous players, with autonomy provided for departures from the recorded script), and hypermedia integration (software for integrating hypermedia data—audio, compressed video, with embedded links—into our geometrically described virtual world). All of this software serves as the base for the fully detailed, fully interactive, seamless environment of the third phase of virtual world development. We discuss the development of such software by describing how a real system, the NPSNET virtual world, is being constructed.