Weight gain and obesity are major risk factors for conditions and diseases ranging from insulin resistance and type 2 diabetes mellitus to atherosclerosis and the sequelae of nonalcoholic fatty liver disease. A chronic, subacute state of inflammation often accompanies the accumulation of excess lipid in adipose tissue and liver (hepatic steatosis), evidenced by changes in both inflammatory cells and biochemical markers of inflammation. These changes can be seen in the involved tissues and systemically, in terms of elevated circulating levels of inflammatory markers. The link between obesity and inflammation has therefore raised the important question of whether obesity-induced inflammation plays a pathogenic role in the development and progression of these disorders. We review the rapidly expanding body of animal and clinical data that support potential roles for inflammation in the pathogenesis of insulin resistance and type 2 diabetes mellitus.