Obesity is associated with an increased risk of developing insulin resistance and type 2 diabetes mellitus (T2DM). In obesity, the adipose cell releases non-esterified free fatty acids, hormones, adipocytokines, and other substances that are involved in insulin resistance. Under normal conditions, the pancreatic islet β cells increase production of insulin sufficiently to maintain normal blood glucose concentrations despite insulin resistance. However, in genetically predisposed patients, the β cells eventually become dysfunctional and T2DM develops. The development of T2DM can be delayed or sometimes prevented in individuals with obesity who are able to lose weight. Weight loss can be achieved medically with behavioral therapies that combine diet and exercise treatment or with behavioral therapies combined with weight-loss medications or weight-loss surgery. In this article, we summarize the evidence of obesity management in treating T2DM and prediabetes.