Understanding the pathogenesis of obesity is now more important than ever, given the remarkable world-wide epidemic. This paper explores the potential role of core temperature in energy balance, and develops the hypothesis that basal temperature and changes in the temperature response in various situations contribute to the enhanced metabolic efficiency of the obese state. The argument is based on the important contribution that heat production makes in establishing the basal or resting metabolic rate, as well as on an analysis of the adaptive role played by changes in temperature in response to environmental challenge. If this hypothesis is validated, new therapeutic approaches may ensue.