The Roux-en-Y Gastric bypass has been the gold standard in overweight surgery for a long time. The procedure is carried out laparoscopically and works by significantly reducing the stomach (gastric pouch) and shortening the resorption distance of the small intestine. The duodenum is completely bypassed by the food passage. The result is a reduced food intake and a reduced metabolism in the small intestine. In order to achieve a satisfactory long-term weight loss and to avoid malnutrition, a lifelong follow up is mandatory.