This paper considers the use of whole-body calorimetry in a wide context, as one of a range of techniques available for advancing our knowledge of the regulation of energy metabolism and growth. This integrated approach should lead to a greater depth of understanding by investigating the mechanisms by which responses of the whole animal depend on events at the tissue, cellular and molecular levels. Two critical stages of mammalian development, the perinatal and early postnatal periods, are discussed. Particular attention is paid to skeletal muscle and the extent to which myofibre differentiation and hypertrophy, and hence muscle function and energetic efficiency, can be modified by nutrition and the thermal environment. In view of the crucial roles of thyroid hormones, growth hormone and insulin-like growth factor-I in regulation of metabolism and growth, the role of the endocrine system is also considered. These studies have important implications for survival and optimal health of humans and other animals.