The extraordinary observation that bariatric surgical interventions not only worked but the effect was durable was perhaps the greatest moment in the “history of obesity”, certainly for the management of obesity. But it was an observation that was largely missed; it was well ahead of its time, and it challenged the most fundamental societal attitudes and beliefs about obesity. Gastro-intestinal procedures, some with very simple changes, could transform the severely obese ill person into a leaner healthier functional individual who never looked back. Was it curing the sloth and gluttony, strengthening the intellect and the character, and providing courage? Or was it effectively treating a disease of disordered regulation of energy balance? Bariatric surgery demonstrated for the first time that there was an effective therapy in humans, but it would take almost 50 years before the critical questions of just how it worked would adequately be scientifically addressed. The process of investigation was actually the reverse of the familiar steps used in developing most novel therapies – “This surgery clearly works in humans, but we need definitive proof so let’s see if it works in rodents”. The procedures do work, and this model is now providing mechanistic insights. We are now starting to understand the mechanisms involved in bariatric surgery, and by extension some of the processes involved in the development and treatment of obesity-related metabolic disease.