If energy must be administered to a sick child, whether undernourished or not, the following questions arise: What is the optimum level of energy to be administered? What proportion of macronutrients must be used to satisfy the exogenous energy to be administered? How can the response to the energy supplementation be controlled? This revision seeks to clarify the factors to be taken into account in the light of the clinician's estimate of the child's energy requirements and, in particular, those of the sick child or the child undergoing parenteral nutrition. For this we describe the components of the child's energy balance and report that it is necessary to be aware of the different areas of energy expenditure, and the way of assessing them. We also describe the factors which are decisive in the well and the sick child's energy expenditure. It is seen that the individual's body composition and metabolic state are among the most important factors when it comes to determining the level and quality of energy to be administered to a child for optimal growth and health. Therefore, the level of malnutrition, the composition of the food received and/or the form of its administration, the physical activity and the type of pathology of the child are important factors in determining the energy expenditure and, therefore, the calorie requirements. While there are numerous studies of these problems, there are still new and promising areas of research.