Designed for the student of urban simulation, this paper offers definitions of "models"--conceptual or mathematics formulations of concrete things or of abstract relations between them--and of "simulation"--an act or process, conducted either by persons or devices, by which a model or a hierarchy of models is made to imitate reality. Six basic model dimensions are outlined and described, and operational definitions for the primary simulation process are provided. The classification of games and game components and the role of computers in simulation are discussed. An outline of the uses and applications of simulation for the urban planner concludes the paper. (RH)