Obesity is a systemic illness that affects virtually every organ system in the body. A presurgical psychiatric evaluation has been advocated as part of a multidisciplinary approach to bariatric surgery. This evaluation seeks to determine the patient’s capacity to understand the risk and benefits of surgery and to appreciate the consequences of surgery. Given the comorbidity of psychiatric illness in the obese, the evaluation also screens for psychopathology, with particular attention paid to disorders of eating behavior. In the presurgical psychiatric evaluation a robust knowledge base concerning the various aspects of weight-loss surgery is essential.