This chapter presents an innovative approach for simulating crime events and crime patterns. The theoretical basis of the crime simulation model is routine activities (RA) theory. Offenders, targets and crime places, the three basic elements of routine activities, are modeled as individual agents. The properties and behaviors of these agents change in space and time. The interactions of these three types of agents are modeled in a cellular automaton (CA). Tension, measuring the psychological impact of crime events to human beings, is the state variable of the CA. The model, after being calibrated by using a real crime data set in Cincinnati, is able to IDEA GROUP PUBLISHING This chapter appears in the book, Geographic Information Systems and Crime Analysis, edited by Fahui Wang. © 2005, Idea Group Inc. 701 E. Chocolate Avenue, Suite 200, Hershey PA 17033-1240, USA Tel: 717/533-8845; Fax 717/533-8661; URL-http://www.idea-group.com ITB10850 198 Liu, Wang, Eck, and Liang Copyright © 2005, Idea Group Inc. Copying or distributing in print or electronic forms without written permission of Idea Group Inc. is prohibited. generate crime patterns similar to real patterns. Results from experimental runs of the model conform to known criminology theories. This type of RA/ CA simulation model has the potential of being used to test new criminology theories and hypotheses.