The study and understanding of the nutritional budget of any animal, including fish, is an essential step for attaining a balanced diet suitable for a particular physical environment. The main aim of current research in the field of fish nutrition is to obtain diets that fish can use to the greatest advantage. This has the double benefit of increasing the profitability of the harvest and at the same time of minimizing polluting wastes. This overview covers the methods and techniques used to determine the different components of the energy budget in both laboratory and field conditions, with special emphasis on the metabolic rate, for which results obtained in the laboratory are given. Finally, the development of descriptive-predictive mathematical models with a bioenergetic focus is analyzed.