Morbid obesity, a condition designated by a body mass index (BMI) ≥ 40 kg/m2 is a major public health problem throughout the world, and is a condition linked to significant morbidity and mortality due to associated co-morbid conditions which include cardiac disease, diabetes mellitus type 2, obstructive sleep apnea, hypertension, dyslipidemia, arthritis, infertility and some forms of cancer. Whereas anti-obesity medications and dietary manipulations result in very limited long-term positive outcomes, surgical treatment of morbid obesity induces long-lasting body weight loss and resolution or improvement of most associated medical conditions, particularly type 2 diabetes mellitus. Several surgical procedures are currently available, including Roux-en-Y gastric bypass (RYGB), biliopancreatic diversion (BPD), vertical banded gastroplasty and adjustable gastric banding. These operations may be performed using laparoscopic techniques to minimize perioperative morbidity and improve postoperative recovery time. In spite of the wide clinical experience with bariatric surgery, the mechanisms underlying the effects of some of these procedure, particularly RYGB, are incompletely understood. Elucidating these mechanisms has become a priority because such knowledge may help understand the relationship between gastrointestinal physiology and insulin resistance as well as possibly identify new targets for the development of novel anti-obesity and antidiabetic medications. This article reviews the current operative techniques for morbid obesity, their indications and outcomes. We also examine available data and hypothesized mechanisms mediating the effects of RYGB.