Personality disorders have for many years been on the fringe of psychiatry, with considerable doubts expressed about the usefulness, implications and validity of the concept. It is argued here that developments in the past few years have brought personality disorders into the mainstream of psychiatric practice. In particular, the recognition that personality function can be separated usefully from clinical symptoms, and that both mental state and personality can be disordered simultaneously, has led to better assessment and understanding. Advances in the classification, epidemiology, treatment and prognosis of personality disorders show that these conditions are common, extensive in their pathology, and cause much suffering. They cannot be ignored or dismissed as peripheral to psychiatry for they are an essential part of good psychiatric practice.