The effects of mental disorder are apparent and pervasive, in suffering, loss of freedom and life opportunities, negative impacts on education, work satisfaction and productivity, complications in law, institutions of healthcare, and more. With new editions of the 'bibles' of psychiatric diagnosis - the ICD and DSM - under development, it is timely to take a step back and re-evaluate exactly how we diagnose and define mental disorder. This new book by Derek Bolton tackles the problems involved in the definition and boundaries of mental disorder. These problems are evident now in many contexts: in the diagnostic manuals themselves, in epidemiological estimates of prevalence, in distinguishing normal sadness from depressive illness, for example, or childhood temperamental traits from developmental psychopathology, and in mental health legislation and criminal law. In many ways these problems are contemporary expressions of those identified in the heated debates surrounding psychiatry in the 1960s and 1970s: does psychiatry pathologize what is really normal life suffering? Is mental illness really social deviance, not a proper domain of medicine? Is psychiatry really a form of social control? However, these original problems have been transformed by crucial developments over the past few decades, and the book seeks to update the position taking them into account. The last few decades have seen the closing of the asylums and the appearance of care in the community: mental disorder is now in our midst, intensifying the problems of the '60s and '70s. Attempts have been made to define clearly a concept of mental disorder that is truly medical as opposed to social, inevitably relying on the distinction between human nature and culture. In the science, there is increasing evidence that this distinction is unviable, and accumulating evidence that there is no clear line between what is normal in the population and what is abnormal. What is Mental Disorder? reviews these various crucial developments and their profound impact for the concept and its boundaries in a provocative and timely book.