Research on comorbidity among psychological disorders is relatively new. Yet, comorbidity data have fundamental significance for classification and treatment. This significance is particularly apparent in the anxiety disorders, which, prior to DSM-III-R, were subsumed under disorders considered more significant (e.g., psychotic and depressive disorders). After considering definitional, methodological, and theoretical issues of comorbidity, data on comorbidity among the anxiety disorders are reviewed as well as data on comorbidity of anxiety disorders with the depressive, personality, and substance use disorders. Treatment implications are presented with preliminary data on the effects of psychosocial treatment of panic disorder on co-morbid generalized anxiety disorder. Implications of comorbidity for research on the nature of psychopathology and the ultimate integration of dimensional and categorical features in our nosology are considered.