Weight gain and obesity are driving the global epidemic of type-2 diabetes through metabolic and inflammatory pathways that cause insulin resistance and impair pancreatic β-cell function, the two important factors that are directly responsible for the development of this disease in susceptible populations. Lifestyle methods and modest weight loss are powerful at preventing and managing type-2 diabetes, but sustaining substantial weight loss is problematic. Bariatric surgery provides exceptional sustained weight loss and remission of type-2 diabetes in 50–85% of subjects, especially if treated early before irreparable β-cell damage has occurred. In addition, there is substantial evidence that bariatric surgery provides additional comorbidity and quality-of-life improvements and reduces mortality in patients with type-2 diabetes. There is an association between the extent of weight loss and remission of type-2 diabetes. Diversionary bariatric procedures such as gastric bypass and biliopancreatic diversion induce a rapid non-weight-loss-associated improvement in glycemic control. Several mechanisms have been proposed for this exciting and novel effect that may provide key insights into the pathogenesis of type-2 diabetes. A range of novel surgical, endoluminal procedures/devices, and pharmacologic therapies are likely to evolve when we better understand how bariatric surgery enables long-term changes in energy balance and non-weight-related metabolic improvements. Bariatric surgery should be considered for adults with BMI ≥ 35 kg/m² and type-2 diabetes, especially if the diabetes is difficult to control with lifestyle and pharmacologic therapy. Although all bariatric procedures produce exceptional results in the management of type-2 diabetes, choice of procedure requires a careful risk–benefit analysis for the individual patient.