Most patients will have clinically significant maladaptive personality traits. These personality traits can substantially complicate the effective treatment of other mental disorders, and they can also be the focus of effective treatment. The assessment of personality disorders is of considerable clinical importance. However, this assessment can also be highly problematic. This article discusses the major issues in the assessment of personality disorders (eg, differentiation from other mental disorders, from normal personality functioning, and gender bias) and summarizes existing research on the convergent and discriminant validity of the semistructured interviews and self-report inventories, which have been developed to improve the reliability and validity of personality disorder assessment.