Most people with alcohol and other drug (AOD) use disorders suffer from co-occurring disorders (CODs), including mental health and medical problems, which complicate treatment and may contribute to poorer outcomes. However, care for the patients' AOD, mental health, and medical problems primarily is provided in separate treatment systems, and integrated care addressing all of a patient's CODs in a coordinated fashion is the exception in most settings. A variety of barriers impede further integration of care for patients with CODs. These include differences in education and training of providers in the different fields, organizational factors, existing financing mechanisms, and the stigma still often associated with AOD use disorders and CODs. However, many programs are recognizing the disadvantages of separate treatment systems and are attempting to increase integrative approaches. Although few studies have been done in this field, findings suggest that patients receiving integrated treatment may have improved outcomes. However, the optimal degree of integration to ensure that patients with all types and degrees of severity of CODs receive appropriate care still remains to be determined, and barriers to the implementation of integrative models, such as one proposed by the Institute of Medicine, remain.