Aggression consumes important amounts of energy (e.g., in fish the effort of "routine" social life may be as costly as life-long forced swimming at moderate speeds). In fish the amount of energy spent and the metabolic compartment mobilized seem to depend on the length of cohabitation, the number of contestants and the result of the fight. In mammals, metabolic preparations for fights were shown. The fights cause elevations of both body temperature and metabolic rate, as well as important changes in carbohydrate and lipid metabolism. There are evidences which show that the energetic aspects of aggressive behavior have a significant impact on the behavioral tactics and survival chances in free living animals. The relevance of these studies to game theoretical analyses and to practical aspects of the aggression-energy metabolism interrelationship are also outlined. Although many details of the phenomenon are known, important issues have to be clarified, among them the possible neuroendocrinologic co-regulation of this behavior and of its energetic background.