Obesity is common in patients with mental illness. Weight-loss surgery, known as bariatric surgery, is becoming a familiar intervention for treating people who are morbidly obese and for whom other weight-reduction methods have failed. This article offers guidance for mental health professionals on the assessment and management of patients with mental illness undergoing such treatment. Assessment is of the patient's suitability for surgery, taking into account their mental health diagnosis, expectations, knowledge and insight into the psychological impact of surgery, and ability to address and cope with lifestyle changes before and after surgery. The patient's capacity and ability to cooperate and engage with services are also assessed. Potential risks and complications of bariatric surgery and how the weight-loss procedures may affect patients' mental health and management of their medication are addressed.