This paper explores the interpretative practices of the police when carrying out
stops and searches. Drawing on ethnographic research carried out in England and
Wales, a conceptual framework is developed to understand how officers become suspicious and decide to initiate encounters. It is argued that officers use tacit knowledge to make sense of new situations and determine what action to take, and base their suspicions on signals they come across on patrol. In examining these social processes, the paper shows that categories and stereotypes are central to decision-making which result in a police focus on the socially marginal. Looking beyond the immediate influences on officer action, the paper also explores the role played by the law in regulating police practice