Automatism is a defence in criminal violence that raises some critical issues about the role of the expert witness in the determination of guilt and innocence. The recent Australian case of R V Leonboyer illustrates some of these issues including psychiatrists and psychologists going beyond their area of expertise, establishing reasonable doubt on the basis of possibilities rather than probabilities and failing to differentiate between automatic behaviour and its causes. Though there may be a necessity to utilise the experience of psychologists and psychiatrists to explain the complex issues arising from the defence of automatism, there are particular problems in using such expert witnesses in this under-researched and complex area.