This is a book about the role that psychological impairment should play in a theory of criminal liability. Criminal guilt in the Anglo-American legal tradition requires both that the defendant committed some proscribed act and did so with intent, knowledge, or recklessness. The second requirement corresponds to the intuitive idea that people should not be punished for something they did not do 'on purpose' or if they 'did not realize what they were doing'. Unlike many works in this area, this book addresses the automatism and insanity defences by examining the types of functional impairment that typical candidates for these defences actually suffer. What emerges is a much wider conceptual framework that allows us to understand the significance of psychological states and processes for the attribution of criminal responsibility in a manner that is logically coherent, morally defensible, and consistent with research in psychopathology.