Bariatric surgery provides substantial, sustained weight loss and major improvements in glycaemic control in severely obese individuals with type 2 diabetes. However, uptake of surgery in eligible patients is poor, and the barriers are difficult to surmount. We examine the indications for and efficacy and safety of conventional bariatric surgical procedures and their effect on glycaemic control in type 2 diabetes. How surgical gastrointestinal interventions achieve these changes is of great research interest, and is evolving rapidly. Old classifications about restriction and malabsorption are inadequate, and we explore understanding of putative mechanisms. Some bariatric procedures improve glycaemic control in people with diabetes beyond that expected for weight loss, and understanding this additional effect could provide insights into the pathogenesis of type 2 diabetes and assist in the development of new procedures, devices, and drugs both for obese and non-obese patients.