There are a number of differences between the body composition of children and adults. Body composition measurements in children are inherently challenging, because of the rapid growth-related changes in height, weight, fat-free mass (FFM) and fat mass (FM), but they are fundamental for the quality of the clinical follow-up. All body composition measurements for clinical use are ‘indirect’ methods based on assumptions that do not hold true in all situations or subjects. The clinician must primarily rely on two-compartment models (that is, FM and FFM) for routine determination of body composition of children. Bioelectrical impedance analysis (BIA) is promising as a bedside method, because of its low cost and ease of use. This paper gives an overview of the differences in body composition between adults and children in order to understand and appreciate the difference in body composition during growth. It further discusses the use and limitations of BIA/bioelectrical spectroscopy (BIA/BIS) in children. Single-frequency and multi-frequency BIA equations must be refined to better reflect the body composition of children of specific ethnicities and ages but will require development and cross-validation. In conclusion, recent studies suggest that BIA-derived body composition and phase angle measurements are valuable to assess nutritional status and growth in children, and may be useful to determine baseline measurements at hospital admission, and to monitor progress of nutrition treatment or change in nutritional status during hospitalization.