This article sets out the complicated and confused law on automatism and identifies the role of the psychiatrist, including paradoxically a role in cases of non-psychiatric disorder where the law requires evidence from a doctor approved under section 12 of the Mental Health Act. Legal definitions of automatism are introduced. The internal/external distinction, evidential burden, burden of proof, standard of proof, prior fault, intoxication and the degree of impairment illustrate how the courts limit the defence. Detailed accounts are given of cases in which the defence of automatism has been based on psychiatric disorder and on the effects of psychotropic drugs. Suggestions are made for approaches to assessment and medicolegal reporting.