In the 1980s clinicians were becoming increasingly aware of the clinical challenges presented by patients with both a mental illness and a substance use disorder. With deinstitutionalization, large numbers of patients who formerly would have been institutionalized were living in the community and had relatively easy access to alcohol and drugs. In this month's issue we continue to examine important developments in psychiatry over the past 50 years by reprinting an article from the October 1989 issue on assessment and classification of patients with psychiatric and substance use disorders, written by Anthony F. Lehman, M.D., and his colleagues (see page 1119). In a commentary, Robert E. Drake, M.D., Ph.D., and Michael A. Wallach, Ph.D., discuss how an understanding of dual diagnosis has developed over the past ten years, along with a growing recognition of the links between substance use disorders and poor outcomes for patients with chronic mental illness (see page 1126)…. In the Taking Issue column, Max Schneier, J.D., says that appropriate treatment depends not on subgrouping of dual diagnosis patients but on careful assessment leading to individualized clinical and psychosocial interventions (see page 1079).