Stalking is a particular type of interpersonal aggression that is difficult to define because it incorporates a range of unwanted behaviors over a protracted period of time that often appear routine and harmless when considered on an incident-by-incident basis. Defining stalking is further complicated because people’s perceptions are integral to determining whether a particular course of conduct constitutes stalking, whether victims identify their own experiences as stalking, and whether support networks and law officials identify other people’s experiences as stalking. This chapter outlines the difficulties associated with defining and legislating against stalking, and reviews literature examining the influence of various personal and situational characteristics on perceptions of stalking. It then considers how perceptions differ from reality and why these differences matter.