NEXUS is a novel simulation environment designed for modeling large neural systems. The simulator allows the user to incorporate complex functional properties and symbolic processing at the level of the individual unit. In addition, NEXUS takes advantage of the principle of topographic map organization, found throughout the mammalian nervous system, to facilitate the modeling and design process. An easy-to-use graphical interface allows the user to interactively build and test models. This paper describes the principles underlying the NEXUS design and its advantages over other current simulation approaches.