The third edition of the textbook presents psychiatry as a medical specialty. The application of science has transformed much of medicine by providing an understanding of the mechanisms of pathology. The genetic basis of psychiatry guarantees a future for explanation by neuroscience. The book sets the scene for such development by explaining the key issues relating to the patient’s perspective, stigma, the global challenge of mental disorder, practical ethics, and the foundations of psychiatry as phenomenology and a medical discipline. It further explains current controversies around diagnosis, psychopathology, evidence, and drug terminology. The scientific basis of psychiatric aetiology and treatment provide simple introductions to the relevant disciplines that underpin our scientific understanding. Individual disorders are covered in sections that follow the structure of the Diagnostic and Statistical Manual of Mental Disorders, fifth edition (DSM-5). Thus, it follows a clinically led summary of how patients present with psychiatric disorder. There is no denying the current utility of symptom-based diagnoses and the consensus that created the current categories. However, the project of applying neuroscience to psychiatry has not failed, as has sometimes been implied by criticism of DSM-5. For these reasons, chapters have been included on genetics, neurobiological targets, and imaging in the sections of the book focused on specific disorders. Sections have also been included on service provision and forensic psychiatry because these are critical to the context in which psychiatric disorder is managed.