Energy balance to prevent the development of obesity is dependent on energy expenditure. Although physical activity is the dominant mechanism for dissipating excess energy, a system of thermogenesis that evolved to protect the body from hypothermia is based upon the uncoupling of oxidative phosphorylation in brown adipocytes by the mitochondrial uncoupling protein (UCP1). It has been shown that upregulation of UCP1 by genetic manipulations or pharmacological agents can reduce obesity and improve insulin sensitivity. Recent evidence has shown the existence of two sources for brown adipocytes, one appearing as discrete brown fat depots during fetal development and the other appears during post-natal development as diffuse populations in traditional white fat depots. The latter can be induced by adrenergic stimulation depending on the genetic background of the animals and the nutritional environment. Understanding the biological and environmental factors controlling the expression of these two brown adipocyte populations promises to provide new strategies by which enhanced thermogenesis can be used to reduce obesity.International Journal of Obesity (2008) 32, S32-S38; doi:10.1038/ijo.2008.236.