Polyuria is an important clinical sign that may pass unnoticed to the relatives and to the physician. In this paper, the definition of polyuria and the pathophysiological bases that condition its presence are reviewed. Likewise, the causes of polyuria in pediatric ages are discussed; this allows the clinician to approach the diagnosis in a systematized and organized manner. Finally, the outstanding clinical and laboratory findings of the entities that usually go with polyuria are reviewed; some of them are: neurogenic diabetes insipidus, primary polydipsia, osmotic diuresis, and nephrogenic diabetes insipidus.