This study examines the role of "working rules" that define what officers interpret as suspicious people, places, and situations. Data were drawn from observational studies of police decisionmaking in Savannah, GA and Miami-Dade, FL. Current theory and research on the use of police discretion and biased policing is focused on the decision to stop, search, or arrest a suspect. Only a few studies focus on processes through which police determine behaviors to be suspicious that influence them to initiate official police action. An analysis of the "working rules" used by officers uncovered 12 substantive categories. The article concludes with a discussion of how this information can be useful in formulating training for police departments. Abstract published by arrangement with Sage Journals.