Psychiatry has long struggled with the nature of its diagnoses. The problems raised by questions about the nature of psychiatric illness are particularly fascinating because they sit at the intersection of philosophy, empirical psychiatric/psychological research, measurement theory, historical tradition and policy. In being the only medical specialty that diagnoses and treats mental illness, psychiatry has been subject to major changes in the last 150 years. This online resource explores the forces that have shaped these changes and especially how substantial ‘internal’ advances in our knowledge of the nature and causes of psychiatric illness have interacted with a plethora of external forces that have impacted on the psychiatric profession. It includes contributions from philosophers of science with an interest in psychiatry, psychiatrists and psychologists with expertise in the history of their field and historians of psychiatry. Each chapter is accompanied by an introduction and a commentary.