More than any other defence in the criminal law, the insanity defence has, and continues to be, the subject of heated debate. Yet too little is known about how the insanity defence operates in different jurisdictions, including in the United Kingdom and Ireland. In this book, Mackay and Brookbanks, and their team of expert contributors, explore the theory and practice around the insanity defence and analyse its diverse influence and manifestations across a wide range of common law and civil law jurisdictions. Typically, the insanity defence, as exemplified in the M'Naghten Rules, represents a foundational aspect of criminal responsibility, although in some jurisdictions it serves only to define degrees of mental capacity. However, what all jurisdictions have in common is the high and increasing incidence of mental illness and impairment challenging existing constructions of an exculpatory rule. This book explores in detail the origins and operation of the M'Naghten Rules as well as the eclectic nature of the insanity defence, its highly variable linguistic expression, and the diverse social policy mandates it seeks to embrace. The Insanity Defence will reinvigorate the debate about the defence by discussing both its theoretical basis and exploring how different jurisdictions approach the insanity plea, not only in relation to an appropriate test and how it operates, but also from the perspective of disposal and how those who use the insanity defence successfully are dealt with. This book will be of interest to researchers, academics, and advanced students with an interest in criminal law internationally, as well as to those involved in the development of policy and legislation.