As most diseases can have nutritional consequences, the assessment of nutritional status may help to detect an underlying disease, to identify nutritional disorders related to a given disease, and to quantify the impact of nutritional therapy. The aims and methods used for nutritional assessment depend on the circumstances in which nutritional assessment is performed. Whatever the context or aim, nutritional status is assessed through a simple, mainly clinical approach, based on the past history, dietary intake, auxological analysis, anthropometric measurements, body compartment and biological parameters. Accurate techniques for measuring body compartments are available in children such as dual-energy X-ray absorptiometry to assess fat body mass or bioelectrical impedance analysis for body water and lean body mass. Measuring energy expenditure allows for a more accurate monitoring of the patient's energy needs and decreases the risks associated with underfeeding or overfeeding. In clinical practice, the analysis should be longitudinal and take into account situations carrying a risk of malnutrition. Preventive use of nutritional assessment allows nutritional support to be introduced in a timely fashion, thereby avoiding morbidity/mortality and limiting the long-term impact of malnutrition on growth and development.