Parents and professionals caring for people with autism sometimes encounter baffling, challenging behaviour -- non-compliance, aggression, or repetitive actions -- which they don't know how to stop. The key to resolving problem behaviour is to first determine its purpose by conducting a functional behaviour assessment (FBA). In this second edition updated with new research, case studies, and anecdotes, the authors explain how clinicians conduct FBAs: measuring behaviour, observing behaviour and collecting data, testing the hypothesis about the behaviour's function. And now, there's new material about how to do FBAs outside of clinical settings -- at school, in the community, and at home -- making this strategy more accessible to parents and teachers in daily life.