Animal production systems have changed dramatically over the last two decades. Knowledge of energy metabolism and environmental physio- logy has increased as appears from many textbooks on these disciplines. The contents of the symposia on energy metabolism of farm animals show this and they have initially focussed on feed evaluation and later on com- parative aspects of energy metabolism. They show part of the progress being made. Application of knowledge of energy metabolism for animals has a long history since Lavoisier. In addition to this, studies about the environ- mental requirements of animals have shown that we are still far from ac- curate assessment of these requirements in terms of nutrients and ener- gy. I n model studies on energy metabolism researchers have recognized the interaction between the environment and the energy requirements of animals. Estimation of energy requirements has been done in physiolo- gical, physical and behavioural studies. The impact of conditions as en- countered by animals in various production systems has been approached from different viewpoints related to these different disciplines. In addi- tion, various kinds of infections (bacterial, parasitic: subclinical, clini- cal) have been evaluated only recently with regard to their effect on pro- tein and/or energy metabolism and thus on production. People working in the field of feed evaluation have defined how che- mical and physical properties of nutrition infiuence energy to be derived for maintenance and production.