The subject of this article is the measurement of energy expenditure and energy balance under field conditions. The objectives of the study are to describe the health status of a community, to understand the regulation of energy balance, and to answer questions on human variation and adaptation in an ecological context. The first part of the article explains some of the steps taken in the choice and implementation of a protocol, and the second part discusses the aims and validity of such field research. It is hoped that two important contributions of this discussion are to give a quantitative appreciation of the levels of energy balance in an ecological context and to offer a perspective to answer some theoretical questions on human variation and adaptation to local environments. (Rajeev Ranjan Singh 2007-02-08)