Widespread use of cocaine by methadone clients is undermining the effectiveness of methadone treatment programs in reducing illicit drug use, decreasing criminal behavior, and slowing the spread of the human immunodeficiency virus (HIV). In response, methadone programs are implementing a range of behavioral interventions to manage this growing problem. Many of these interventions, however, have yet to be evaluated for effectiveness for reducing cocaine use among methadone clients. Interventions that are effective for cocaine use in the general population may not be as effective with cocaine users in methadone programs because these clients differ from other cocaine users in ways that are likely to affect how they respond to the interventions. This paper reviews the literature on the significance and scope of the problem of cocaine use by methadone clients and on the behavioral interventions that have been evaluated for these clients.