Stalking and harassment are not discrete entities, but patterns of behaviour in which individuals engage for different reasons and with different objectives. By definition, stalking causes psychological harm in that it gives rise to fear and apprehension. That fear may be of physical aggression or simply that the intrusive harassment will not end. The stalking may be accompanied by direct or indirect threats, or threat may be inferred from the behaviour of the stalker. In a proportion of cases, stalking involves physical violence, either to persons or property, or both. Yet, unlike some of the phenomena discussed in this handbook, stalking is not always a form of conscious aggression. Where, for instance, the dominant emotion is love and the harassing activities involve the sending of letters and gifts, individuals are unlikely to have any hostile intent, even if the effect on the victim is one of fear and alarm. This contrasts, for instance, with the clear, malicious intent of a resentful person who stalks with the specific aim of causing harm. Consideration of motivation, together with previous relationship (if any) and psychopathology, is central to the understanding of individual cases, to the assessment and management of risk, and to the treatment of those engaging in stalking or harassment. This chapter sets out the characteristics of the phenomenon of stalking, and illustrates how research findings concerning risk have been incorporated into specialist approaches to risk assessment and management.