Obesity results when energy In lake exceeds energy expenditure. Naturally occurring genetic mutations, as well as ablative lesions, have shown that the brain regulates both aspects of energy balance and that abnormalities in energy expenditure contribute to the development of obesity. Energy can be expended by performing work or producing heat (thermogenesis). Adaptive thermogenesis, or the regulated production of heat, is influenced by environmental temperature and diet. Mitochondria, the organelles that convert food to carbon dioxide, water and ATP are fundamental in mediating effects on energy dissipation. Recently there have been significant advances in understanding the molecular regulation of energy expenditure in mitochondria and the mechanisms of transcriptional control of mitochondrial genes. Here we explore these developments in relation to classical physiological views of adaptive thermogenesis.