In recent years, adipose tissue has become recognized as a highly metabolically active organ. In 1994, the hormone leptin was identified, a peptide almost exclusively secreted by adipose cells and with receptors both in the hypothalamus and peripheral tissues. Leptin has a number of actions both in relation to signalling satiety and altering energy metabolism. The identification of a rare family with leptin deficiency, extreme obesity and insulin resistance was followed by treatment of two children with recombinant leptin and successful loss of weight. However, in the majority of non-leptin-deficient obese individuals, circulating leptin levels are high and correlated with body fat mass, suggesting that leptin resistance may play a role in human obesity. Further work is needed to establish the exact role of this hormone in energy homeostasis.