Fundamental to appropriate nutritional prescription is an understanding of the conditions necessary for growth that include the requirements for energy in health and illness. Energy requirements need to be met by the dietary intake to prevent weight loss. A positive energy balance will result in weight gain. Energy requirement includes several components; the largest is the basal metabolic rate, although physical activity level and the energy needs of growth are important components. All aspects of energy metabolism are likely to be influenced by illness and impact on energy balance. Changes in dietary intake and physical activity are observed clinically but poorly described in most childhood illnesses. Changes in metabolic rate are poorly described in part owing to methodological problems. This review explores changes in energy expenditure associated with health and disease, highlights the lack of evidence underpinning this aspect of practical nutritional support and provides the clinician with a guide to the factors involved in estimating energy requirements, emphasising the importance of measuring the child's response to nutritional support.