Artificial processes for locating urban activities based on simple rules pertaining to local circumstances give rise to complex global patterns that mirror the spatial organization of cities. These systems are called Cellular Automata (CA). They provide a useful means of articulating the way highly decentralized decision-making can be employed in simulating and designing robust urban forms. CA can be easily programmed in a variety of software, and as such provide a suggestive way of exploring actual as well as optimal patterns and plans. This primer provides a pedagogic guide to these ideas and to potential computer applications.