The benefit of RYGB on glycemic control in morbid obese patients (BMI > 35 kg/m2) with T2D is confirmed and the Diabetes Surgery Summit Consensus Conference accepted bariatric surgery as alternative therapy of obesity related T2D. They also consider expanding the indication for bariatric surgery in obese patients with T2D who currently don't meet the international guidelines for bariatric surgery. In this regard it would be helpful to identify preoperatively which T2D patients would benefit from RYGB. Prognostic factors that were described previously are the duration of diabetes, age, preoperatively HbA1c levels and fasting C-peptide. However, the underlying cause of T2D resolution and the prognostic value of these factors are not completely understood. Possibly, T2D resolution depends on the preoperative available beta cell mass and function and increased beta cell failure or reduced beta cell ass might decrease the chance of T2D resolution. Therefore, more information on the effects of bariatric surgery on beta cell function and beta cell mass will be helpful to further understand T2D resolution and the role of the beta cells. Thereby, preoperative beta cell mass might be a prognostic factor of T2D resolution and helpful in identifying obese T2D patients that will benefit from RYGB and expanding the indications for bariatric surgery for those patients. For this purpose reliable, sensitive and specific visualization of living pancreatic beta cells in vivo is important.