Although water is essential for life, its use for medicinal purposes is not universally accepted. We performed a comprehensive review of the literature to determine where the evolving state of knowledge lies regarding the benefits of water as a therapy for renal diseases. In the past two decades, water has emerged as a potential therapeutic agent in nephrolithiasis, chronic kidney disease (CKD), and polycystic kidney disease (PKD) in particular. In nephrolithiasis, the benefit of drinking water beyond that demanded by thirst is a cornerstone of therapy for both primary and secondary disease. In CKD, recent observational studies suggest a strong, direct association between preservation of renal function and fluid intake. In PKD, increased water intake slows renal cyst growth in animals via direct vasopressin suppression, and pharmacologic blockade of renal vasopressin-V2 receptors has recently been shown to be efficacious in retarding cyst growth in PKD patients. Although evidence is lacking to support increased water intake in the general population, available evidence indicates that individuals who are at risk for nephrolithiasis as well as those with CKD and PKD may benefit from 3 to 4 l of urine output each day, a level of excretion that is likely to be safe.