This article discusses the historical underpinnings of psychiatric classification and examines empirical evidence relevant to (a) whether personality disorders are distinct from each other and from normal personality and (b) whether personality disorders should be classified separately from other mental disorders. At the phenotypic level, research evidence strongly supports the use of a dimensional model to delineate personality disorders; evidence about their genotypic representation is less conclusive though still supportive. Neither empirical nor rational arguments indicate strong justification for separating personality disorders from other mental disorders, as has been done in both the third and fourth editions of the Diagnostic and Statistical Manual of Mental Disorders. Distinctions between abnormal and disordered personality are considered, and suggestions are made for more satisfactory diagnostic classificatory schemes.