Behavioral and pharmaceutical intervention to treat obesity and its comorbidities typically results in only a 5-10% weight loss. Thus, bariatric surgery is the most effective obesity treatment with some surgeries resulting in 30% sustained weight loss. Although this degree of weight loss has profound metabolic impact, these surgeries seem to have metabolic effects that are independent of weight loss. In support of this is the clinical literature showing rapid resolution of Type 2 diabetes mellitus (T2DM) that occurs before significant weight loss. To gain a complete understanding of the weight loss-independent effects of bariatric surgery, animal models have been developed. These are becoming more widely implemented and allow the use of pair-fed or weight-matched sham-operated controls in order to gain mechanistic insights into the mode of action of bariatric surgery. Increases in anorectic gut hormones, such as glucagon-like peptide-1 and peptide YY, or decreases in the orexigenic hormone ghrelin have been seen and are implicated as mediators of weight loss-independent actions of bariatric surgery. Changes in nutrient processing and sensing may also have a mechanistic role that is independent of, or that regulates, gut hormone responses to these surgeries. Ultimately, the hope is that understanding the mechanisms of bariatric surgeries will aid in the development of less invasive surgeries or pharmacological therapies that are more specifically, and perhaps individually, targeted at weight loss and/or resolution of T2DM.