Dual diagnosis, which refers to diagnosis and treatment of co-occurring psychiatric and substance use disorders (SUD), is a mental health treatment approach which is only becoming more important to learn about as the recognition of the prevalence of comorbid substance use and psychiatric illness is increasing, and data show that people with serious psychiatric illness are more likely to relapse in the treatment of their substance use. These two issues, of psychiatric disorder and substance use, are intertwined in a complex, and not yet fully elucidated manner, but treatments that address both one’s underlying psychiatric diagnosis and their co-morbid substance use disorder are more likely to have long-term success. Previous treatment models attempted to address the two issues sequentially or in parallel, but as our understanding of the comorbidity develops, it has become clear that integrated treatment, with a multidisciplinary team that works together on both aspects of the dual diagnosis, is the gold standard. Each case requires an individualized approach to find the most effective psychotherapeutic and pharmacologic treatment regimen.