The purpose of this article is to review the current state of knowledge on contributions of nocturnal urine overproduction and overactive bladder to the syndrome of nocturia. We review the recent literature and current state of the art in differential diagnosis, pathophysiology, and classification of nocturia. We found that multiple pathologic factors may result in nocturia, including cardiovascular disease, diabetes mellitus or insipidus, third spacing of fluid, sleep apnea, lower urinary tract obstruction, primary sleep disorders, and behavioral and environmental factors. Thus, nocturia may be attributed to nocturnal polyuria (nocturnal urine overproduction), diminished nocturnal bladder capacity, or both. Distinction between these conditions is made by a simple arithmetic analysis of the 24-hour voiding diary. Understanding the manifold origins of nocturia will lead to rational treatment of specific contributing pathophysiologic factors.