In recent years patients with dual diagnosis have increased in America. `Dual diagnosis` or dual disorder are terms that have been used to indicate the coexistence of a mental health disorder and substance use disorder. Patients with mental disorder are vulnerable to risk for substance use disorders, patients with substance use disorders are also vulnerable to risk for mental disorders. Even though there have been few people treated for dual diagnosis in Korea, there is need for concern about this diagnosis because a dual diagnosis makes the individual more incapicitated. In Korea, several research papers have identified that personality disorders or depression coexisting with alcoholism. There are several possible relationships between substance abuse and psychiatric disorders. Substance abuse may induce, worsen, or diminish psychiatric symptoms, complicating the diagnostic process. Sometimes, the symptoms of coexisting psychiatric disorder may be misinterpreted as poor or incomplete recovery from drug addiction. In addition, psychiatric disorders may interfere with the patients` ability and motivation to participate in addiction treatment. This article provided an overview of characteristics and assessment based on a biopsychosocial model. Indeed, the treatments for dual diagnosis are very complex and difficult and the prognosis for this disorder is very poor. In this article, pharmacologic and psychosocial treatment model also is provided. Finally, it is suggested that nurses who meet patients with mental disorders or substance use disorders can have an important role in detecting or preventing the occurrence of dual diagnosis.