Issues: Comorbidity between anxiety and substance use disorders is common, yet it is poorly understood and poorly treated. Approach: Narrative literature review. PsycINFO and Medline databases were searched for clinical trials of anxiety and substance use disorders using clinical queries for 2005-2009. Key Findings: There are few well-conducted treatment outcome trials for comorbid anxiety and substance use disorders. Some recent (2005-2009) outcome literature has focused on specific mechanisms (anxiety sensitivity and tension reduction alcohol expectancies) that may underlie comorbidity between anxiety and substance use disorders and may lead to more targeted intervention. Implications and Conclusion: The research base for understanding and treating comorbid anxiety and substance use disorders needs to be broadened. In particular research is needed with a focus on: (i) specifying particular comorbid relationships between anxiety and substance use disorders; (ii) the mechanisms that may underlie and maintain those relationships; and (iii) well-conducted evaluations of treatments that target those mechanisms.