164 Authors’ Note: We would like to thank participants of the Academy of Criminal Justice Sciences 44th Annual Meeting and Paul J. Brantingham for helpful comments. The usual disclaimer applies. Criminal Justice Policy Review Volume 19 Number 2 June 2008 164-180 © 2008 Sage Publications 10.1177/0887403407311591 http://cjp.sagepub.com hosted at http://online.sagepub.com Crime Prevention and the Science of Where People Are Martin A. Andresen Simon Fraser University, Burnaby, BC, Canada Greg W. Jenion Kwantlen University College, Surrey, BC, Canada Crime prevention initiatives are often conceptualized working at primary-secondary- tertiary (PST) levels. Primary prevention efforts address the underlying social, economic, and physical environmental conditions that generate crime; secondary prevention efforts focus on people, places, and social conditions that are at high risk of crime; whereas tertiary prevention efforts are directed toward already existing and specific crime problems. This article discusses the uses of the ambient population (a 24-hr average estimate of the population present in a spatial area) to better inform crime prevention initiatives within the PST framework. Though the results indicate the ambient population has utility for all three levels of crime prevention, the most immediate use is in tertiary prevention to better understand the nature of areas with a current crime problem. This information is not available from the resident (or census) population because the resident population indicates where people sleep, not where they are. Keywords: ambient population; crime prevention; PST model C rime is a complex phenomenon that occurs when an offender, a victim, and a law converge in time and space (Brantingham & Brantingham, 1981). Despite general classifications of crime (property crime, violent crime, white-collar crime, or nuisance crime), its cumulative impact has many monetary and psychological costs: property loss, insurance, law enforcement, the judiciary, corrections, victim- ization, and safety (Brantingham & Easton, 1998; Sharpe, 2000). As such, crime prevention or crime reduction can have positive effects on society (United Nations Economic and Social Council, 2002). Correspondingly, many evidence-based crime prevention activities (see Sherman, Farrington, Welsh, & MacKenzie, 2002) aspire to prevent the convergence of an offender and a victim in time and space. The definition of crime prevention has eluded broad definitional acceptance by academics (Lab, 1997, 2004; Tilley, 2005), but its abundant use in society continues to be an area that is pursued by academics, practitioners, and governments alike. Aside from the obvious need for crime data in crime prevention activities, other environmental data such as roads, shopping centers, and land use may prove to be