Ultrasonography (US) has largely replaced the intravenous urogram as the first modality for the evaluation of the kidneys in children suspected of having urinary tract abnormalities. Because many renal disorders are associated with changes in the sizes of the kidneys, normative standards for assessing renal size have been developed. These standards rely upon comparison of the renal lengths or calculated volumes or both, with various assessments of overall body size, including body surface area, weight, height, and chronological age. We discuss some of the limitations of US in assessing renal size in children. Practical recommendations are offered for optimizing the measurement and interpretation of sonographic renal sizes in children.