In critically ill children, optimal nutritional intake can improve recovery. During stay in the pediatric intensive care unit (PICU), both underfeeding and overfeeding have to be avoided. The assessment of energy requirements is difficult during critical illness because of lack of devices to measure it. In addition, energy requirements vary during the different phases of critical illness; i.e. the acute, stable and recovery phase. Energy expenditure is affected by the illness itself, but also by the conditions of the PICU such as mechanical ventilation, sedation, fever and the lack of physical activity. During the acute phase, energy intake has to be increased gradually while not exceeding resting energy expenditure (REE). The latter may be estimated using the Schofield equation or ideally be measured using a validated indirect calorimeter. After the acute phase, energy intake has to be increased to enable recovery and growth. The aim of this review is to give an overview of the current knowledge concerning REE in critically ill children and factors that may affect REE. The principles of measurement or calculation of REE will be described. The available evidence will be synthesized and practical recommendations will be provided for guiding energy intake during the different phases of illness.