The purpose of this article is to provide a review of the prevalence, assessment, and treatment of common psychiatric disorders found among patients with opioid dependence. Dependence on opioids can include both persons who are physically dependent on opioids and persons who fulfill the criteria for a syndrome of opioid dependence, such as that found in the Diagnostic and Statistical Manual, fourth edition (DSM-IV). The latter grouping of persons typically abuse illicit opioids, and prevalence of comorbid conditions and approaches in diagnosis and treatment have been studied in these patients. High rates of other psychiatric disorders--both other substance-use disorders as well as non-substance-use psychiatric disorders--have been reported. The most common non-substance-use psychiatric disorders are depressive, anxiety, and personality disorders. When evaluating and planning treatment of opioid-dependent patients with concurrent psychiatric symptoms, it is important to determine if such symptoms are independent of the substance use or substance induced. In the former case, treatment should follow routine clinical practice, whereas in the latter case, treatment stability in substance use should be the first therapeutic step. The presence of a pain condition can further complicate assessment and treatment, as either pain itself or treatments used for pain may produce symptoms that overlap with psychiatric disorders.