Converging evidence from epidemiologic and treatment studies indicate that anxiety disorders and substance use disorders commonly co-occur, and the interaction is multifaceted and variable. Epidemiological studies and investigations within clinical substance abuse populations have found an association between anxiety disorders and substance use disorders. Specific anxiety disorders including generalized anxiety disorder, panic disorder, and post traumatic stress disorder have all been associated with substance use. The association with obsessive-compulsive disorder is less robust, and some research has found a negative association. The risk of nicotine dependence is significantly higher among individuals with an anxiety disorder, and conversely, smoking has been found to be associated with trait anxiety and anxiety disorders. A review of the current literature and the relationship between specific anxiety disorders and alcohol and substance use disorders is discussed in detail. This article, written for social workers in a variety of practice settings, reviews the prevalence, diagnostic, and treatment issues at the interface of substance use disorders and anxiety disorders.