Fatty acids are important oxidative fuel for liver, kidney, skeletal muscle, and myocardium. There has been much interest in the role of fatty acids in the pathogenesis of non-insulin-dependent diabetes because they compete with glucose for oxygen and inhibit whole body glucose disposal via the 'Randle cycle,' Control of lipolysis in adipose tissue determines systemic fatty acid supply. A wide range of hormones and other substances have been recognized as regulators of lipolysis, but insulin and catecholamines appear to be the most important. The regulation of lipolysis, in most circumstances, provides a supply of lipid fuel exceeding the rate of lipid oxidation, requiring reesterification to triglyceride of surplus circulating free fatty acids. Thus, free fatty acid supply is usually not matched to the demand for lipid oxidation, and there is no known mechanism for accurately sensing such demand. This lax regulation may be disadvantageous in conditions such as aging, stress, obesity, and diabetes, where the antilipolytic effect of insulin is impaired and lipolysis is therefore increased. In these conditions, the surfeit of fatty acid may impair glucoregulation. In addition, the excess lipolysis may induce hypertriglyceridemia (via increased very low density lipoprotein production) and thus contribute to atherogenesis. Considerable additional research is needed in order to fully understand both normal lipolytic regulation and the abnormalities of lipolysis which accompany pathological conditions.