Clients with concurrent substance abuse and other psychopathology constitute an often neglected patient population which presents significant assessment and treatment challenges. Proper treatment requires a careful assessment of issues not always addressed in standard substance abuse or mental health treatment settings. Psychologists with expertise in the psychology of addiction and psychopathology are needed to develop and implement effective approaches to working with such patients. Assessment issues which need to be addressed include the nature and severity of both types of problems, but also the presence and nature of interactions between concurrent problems. This document addresses some of the assessment and treatment questions posed by the client presenting with concurrent substance abuse and psychopathology. It sets forth a way of classifying concurrent problems, including a taxonomy of types of interactions between substance abuse and other psychological problems. To facilitate such an assessment, a set of assessment categories is proposed. Common treatment options for persons with concurrent problems are discussed. Optimal treatment strategies for persons falling into the various assessment categories are considered, with a particular emphasis on the treatment of problem interactions. (Author/NB)