The incidence of type 2 diabetes mellitus is rising. It presently affects more than 150 million people worldwide, and 7.5% of the population of Europe suffer from this disease. This is partially explained by an increase in the prevalence of obesity. Less than 10% of the diabetic patients achieve appropriate control of their illness. For over a decade, it has been observed that the resolution of type 2 diabetes is an additional outcome of surgical treatment of morbid obesity. Moreover, it has unequivocally been shown that, postoperatively, diabetes-related morbidity and mortality have significantly declined. This improvement in diabetes control is long lasting, and was well documented postoperatively for at least 16 years. Two procedures, the Roux-en-Y gastric bypass (RYGB) and the biliopancreatic diversion (BPD), are more effective treatments for diabetes than other procedures. They are followed by normalization of concentrations of plasma glucose, insulin and glycosylated hemoglobin in 80-100% of morbidly obese patients. Studies have shown that results return to euglycemia and normal insulin levels occur within days after surgery, long before any significant weight loss occurs. This fact suggests that weight loss alone is not a sufficient explanation for this improvement. Other possible mechanisms effective in this phenomenon are decreased food intake, partial malabsorption of nutrients, and anatomical alteration of the gastrointestinal tract, that incites changes in the incretin system, which in turn, affect glucose balance. Better understanding of those mechanisms may lead to the discovery of new treatment modalities for diabetes and obesity.