This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book discusses issues of defining and measuring aggression to provide a platform for presenting research findings based on different conceptualisations and methodologies. It presents an overview of theories explaining aggressive behaviour, dividing them into approaches that stress the biological foundations of aggressive behaviour and those that explain aggression as the result of psychological processes. The book is concerned with the development of individual differences in aggressive behaviour and the stability of such differences in aggression over the lifespan. It describes an issue that has attracted intense controversy, particularly in public debate and in confrontation between social scientists and the media industry. The book provides a review of issues that belong to the standard repertoire of any introductory text on aggression. It deals with aggression in a domestic context and covers issues such as child abuse, spouse battering, and elder abuse.