Millions of people worldwide suffer from type 2 diabetes mellitus. There are two major factors that play a role in its pathogenesis: insulin resistance and impaired secretion of insulin by beta-cells. Impaired insulin action is closely linked to the phenomenon of obesity. The adipose tissue releases free fatty acids but also hormones and cytokines such as leptin, adiponectin, resistin, TNF-alpha and others. All those particles modify insulin action. This review article summarizes contemporary opinions on the role of obesity in the generation of resistance to insulin and subsequently impaired glucose tolerance as well as overt type 2 diabetes mellitus.