In the 50th Anniversary Issue of the Journal of Animal Science, Hanson (1958) reviewed 50 yr of progress in swine nutrition and emphasized amino acid requirements and protein quality, mineral and vitamin requirements, use of antibiotics and changes in methods of feeding. Advances in knowledge of energy metabolism were not specifically addressed. Beeson (1958) predicted that total digestible nutrients (TDN) would be replaced by digestible and metabolizable energy values and that the energy values of diets would be expressed in kilocalories. Beeson (1958) also suggested that the energy content of feeds would have to be reevaluated. Interest in concise definition of energy requirements and energy utilization was apparent in 1958 with the organization of an International Symposium on Energy Metabolism under the sponsorship of the European Association of Animal Production. Symposia have been held at 3 yr intervals and have provided a continuing stimulus for energy metabolism research. Early studies of energy metabolism of swine were reported by Capstick and Wood (1922), Deighton (1923, 1929), Brierem (1936, 1939) and Brody (1945). NRC (1964) and ARC (1967) adopted the expression of energy requirements of swine in terms of digestible or metabolizable energy. It is evident that the importance of describing diets for swine in terms of energy and research in the area of energy metabolism of swine has greatly increased in the last 25 years. Within this review, it is not possible to cite all of the contributors to the evolution of the current understanding of energy metabolism but an effort is made to summarize the key concepts that have been elucidated in the understanding of energy metabolism of pigs.