Interest in the interface between normality and psychopathology was renewed with the publication of DSM-III more than 20 years ago. The use of a separate axis to classify disorders of personality brought increased attention to these conditions. At the same time, the definition of personality disorder as inflexible and maladaptive traits stimulated interest in the relationship between normal and disordered personality structure and functioning. The evidence suggests that the traits delineating personality disorder are continuous with normal variation and that the structural relationships among these traits resemble the structures described by normative trait theories. Recognition that personality disorder represents the extremes of trait dimensions emphasizes the importance of differentiating normal, abnormal, and disordered personality. It is argued that while abnormal personality may be considered extreme variation, personality disorder is more than statistical variation. A definition of personality disorder is suggested based on accounts of the adaptive functions of personality. Copyright © 2005 John Wiley & Sons, Ltd.