This report provides an overview of ROSS, an object-oriented language currently being developed at RAND. The goal of ROSS is to provide a programming environment in which users can conveniently design, test, and modify large knowledge-based simulations of complex mechanisms. Object-oriented programming languages, and ROSS in particular, enforce a message-passing style of programming in which the system to be modeled is represented as a set of objects and their behaviors (rules for object interaction). This style is especially suited to simulation, since the mechanism or process to be simulated may have a decomposition that maps naturally onto objects, and the real-world interactions between the objects may be easily modeled by object behaviors and object message transmissions. In addition to describing some of the basic ROSS commands and features, the report discusses some software that interfaces directly with ROSS, including a sophisticated screen-oriented editor and a color graphics package. Facilities for browsing among objects and their behaviors are also described, and examples of browsing and editing are presented using SWIRL, a military combat simulation written in ROSS.