Most studies on police decisionmaking agree that police officers use simplification and categorization as necessary ingredients in deciding whether to intervene in a situation. Police work involves many factors which limit the ability to judge possible offenders accurately, such as ambiguous information, an overreliance on negative information, a tendency to rely on recent experiences, and hindsight bias. Using theories from cognitive psychology, the authors outline a model of police perceptions. Patrol officers recognize the world in five categories: the first two are those of normal and nonnormal situations while the other three classify people as dangerous individuals, those likely to cause problems, and the rest. Each category has an associated set of attributes defining its range and content. For example, four attributes categorize people: danger, nonreactive behavior, reactive behavior, and physical characteristics. Because forming new categories and attributes is a difficult and long-term process, it may be preferable to assume that mistakes will occur in police work and with some frequency, rather than contending they happen only rarely. Research studies should address the patrol officer's world view and the accuracy of patrol perceptions and judgments. The paper contains 2 footnotes and 46 references. For related material, see NCJ-86875.