By presenting an interdisciplinary analysis of various factors which inform mental state defences in criminal law, this book provides several practical and robust reform proposals. Three objectives underpin the suggested reform proposals: first, to ensure that an accused will not be held responsible for involuntary conduct even whilst aware of both the nature and quality of such conduct, second, to provide principled means by which to establish the responsibility of an individual in a state of drug-induced psychosis, and third, to ensure that criminal conduct arising from a state of 'impaired consciousness' does not automatically result in an outright acquittal. By exploring appropriate boundaries for the defence of insanity and the doctrine of automatism, this book suggests a consistent and principled approach to the reform of mental state defences. In illustrating the competing demands which must be balanced in order to secure such an approach to the reform of mental state defences, the book will be relevant to all common law countries.