Research on personality traits has made significant contributions to understanding clinical syndromes such as substance abuse and personality disorder. This paper reviews evidence that supports the role of constellations of personality traits and personality disorders that are commonly observed in substance abusers. The potentially overlapping behavioral expression of some personality traits, disorders, and substance use creates challenging measurement issues that can only be resolved through sophisticated assessment strategies. A reliable and valid assessment should depend on a multi-level assessment of personality traits, personality-related problems, and personality disorders involving both self-report and structured clinical interviews. A comprehensive assessment approach should guide the development and evaluation of promising therapeutic strategies and inform revisions to the psychiatric nosology of personality disorders. (C) 2004 Elsevier Inc. All rights reserved.