Starting with a discussion of antisocial personality disorder as a diagnostic construct, this chapter argues that models of mental disorders should focus on symptoms (and individual traits) rather than on flawed diagnostic syndromes. It is argued that interpersonal antagonism, callousness and hostility lie at the core of antisocial personality, which is broader than the constructs – ASPD and psychopathy – with which it is traditionally associated. Paranoia and boredom proneness are considered as key elements of antisocial personality. Assessment of antisocial individuals should consider the three aspects of self: self as social actor, self as motivated agent, and self as autobiographical author, as well as their location on the prosocial-antisocial continuum. Arising from the assessment, a case formulation should seek to articulate the central mechanisms that cause and maintain the individual's main problems and to explain how they are related. When an individual shows violent behaviour it is important that the case formulation identifies the type of violence that is shown and the functions it serves. Consistent with the Good Lives Model (GLM), it is important that positive aspects are included in a strength-based case formulation, covering areas such as work, relationships, accommodation, health and leisure activities.