This article will provide a pathophysiologic basis for the assessment of critically ill children who have developed disorders of urine volume. The anatomical and pathophysiologic causes of oliguria and polyuria are considered. The physiologic basis for the use of urinary sodium and osmolarity as a guide to the assessment of patients with disorders of urine volume are discussed in detail. In addition, guidelines for the management of children with acute renal failure, with particular emphasis on the consideration for nutritional support of these patients, is discussed as a part of the comprehensive approach to this problem. This article emphasizes an understanding of the pathophysiology of salt and water excretion by the kidney as a foundation to the diagnosis and management of patients with oliguria and polyuria.