Personality disorders are common and ubiquitous in all medical settings, so every medical practitioner will encounter them frequently. People with personality disorder have problems in interpersonal relationships but often attribute them wrongly to others. No clear threshold exists between types and degrees of personality dysfunction and its pathology is best classified by a single dimension, ranging from normal personality at one extreme through to severe personality disorder at the other. The description of personality disorders has been complicated over the years by undue adherence to overlapping and unvalidated categories that represent specific characteristics rather than the core components of personality disorder. Many people with personality disorder remain undetected in clinical practice and might be given treatments that are ineffective or harmful as a result. Comorbidity with other mental disorders is common, and the presence of personality disorder often has a negative effect on course and treatment outcome. Personality disorder is also associated with premature mortality and suicide, and needs to be identified more often in clinical practice than it is at present.