Obesity is reaching epidemic proportions in the developed world. In morbidly obese patients only surgical treatment (bariatric operations) leads to a sustained weight loss and cure of comorbidities in the majority of patients. There exist a number of different operations resulting in either a restrictive effect and/or malabsorption, accompanied by a humoral effect which is caused by changes of the different gastrointestinal hormones. It is still unknown which patients needs which operation.