Objective: Alcohol use disorders and other substance use disorders are extremely common among patients with posttraumatic stress disorder (PTSD). This article reviews studies pertaining to the epidemiology, clinical phenomenology, and pathophysiology of comorbid PTSD and substance use disorders. Method: Studies were identified by means of computerized and manual searches. The review of research on the pathophysiology of PTSD and substance use disorders was focused on studies of the hypothalamic-pituitary-adrenal axis and the noradrenergic system. Results: High rates of comorbidity suggest that PTSD and substance use disorders are functionally related to one another. Most published data support a pathway whereby PTSD precedes substance abuse or dependence. Substances are initially used to modify PTSD symptoms. With the development of dependence, physiologic arousal resulting from substance withdrawal may exacerbate PTSD symptoms, thereby contributing to a relapse of substance use. Preclinical work has led to the proposal that in PTSD, corticotropin-releasing hormone and noradrenergic systems may interact such that the stress response is progressively augmented. Patients may use sedatives, hypnotics, or alcohol in an effort to interrupt this progressive augmentation. Conclusions: Vigorous control of withdrawal and PTSD-related arousal symptoms should be sought during detoxification of patients with comorbid PTSD and substance use disorders. Inclusion of patients with comorbid PTSD and substance use disorders in neurobiologic research and in clinical trials will be critical for development of effective treatments for this severely symptomatic patient population.