This introductory chapter is divided into three sections. The first section gives details of some selected incidents of aggression and violence that have occurred in the sport context. Most of these received a high profile in the television and print media. These particular examples have been selected because they illustrate the range and variety of situations that can occur in sport, and because they highlight a number of contentious issues about aggressive and violent behaviour in sport which will be discussed later in this chapter and the rest of the book. The second section is concerned with attempts in mainstream psychology to define aggression and violence in general, and in sport psychology to find satisfactory definitions for aggression and violence in the context of sport. The third section examines the various theoretical explanations that have developed over the years as to why aggression takes place.