In this chapter, I describe a system for monitoring crime frequencies for a set of small areas. The objective is to detect as quickly as possible any increase in any area’s crime frequency, relative to a specified expected frequency. The system uses a cumulative sum approach, cumulating differences between the observed and expected frequencies of crime in each area. The approach is illustrated using 1996 burglary data from Buffalo, which is available by census tract. Computer code associated with the geosurveillance program is provided in the appendix.