Abstract: Co-occurrence of axis I disorders, in particular bipolar and substance disorders, presents a treatment challenge to researchers and clinicians as well as major health concerns for society. Although awareness of these co-occurring disorders has increased in recent years, they are often misdiagnosed or treated separately. Differentiating these psychiatric symptoms and determining whether there is a primary, secondary, or co-occurring disorder are essential for effective treatment. The ultimate goal is to alleviate patient suffering, lower mortality rates, and provide cost-effective treatment. Although pharmacotherapy is a standard form of treatment for bipolar disorder, treatment issues are often raised when treating co-occurring substance disorders. The present study provides a critical review of the literature between 1992 and 2005 on the assessment, diagnosis, and treatment of bipolar and substance disorders. This review focused on books, journal articles, reviews, and dissertations that related to the topic of dual diagnosis for the adult population. This critical review included (a) overview of bipolar and substance disorders; (b) evaluation of the assessment and diagnosis of bipolar disorder and substance disorders; (c) an examination of differentiating primary, secondary, or co-occurring symptoms of bipolar and substance disorders; (d) an examination of potential pitfalls resulting in misdiagnosis; (e) an evaluation of various treatment methods for bipolar and substance disorders; and (f) an evaluation of the efficacy of these treatment methods. The results of this study indicated a need for further research in the simultaneous treatment of co-occurring bipolar and substance disorders. Due to the variance in symptomology and diagnostic criteria used in the studies, it was difficult to compare and contrast outcomes. The complexity of diagnosis was highlighted, along with the potential for misdiagnosis. Two particular strengths were found in the literature: (a) increased awareness of the movement of bipolar disorder toward a bipolar spectrum, and (b) the identification of overlapping symptomology between disorders that often confuses accurate diagnosis. A weakness found throughout the studies was a lack of simultaneous treatment for co-occurring bipolar and substance disorders, including those that the studies often identified as treating co-occurring disorders. Therefore, actual treatment of co-occurring bipolar and substance disorders was significantly limited.