The kidneys play a critical role in the homeostasis of body fluid tonicity and effective circulating volume. Renal homeostatic mechanisms are frequently challenged in acutely ill people. Fluid depletion causing hypovolemia may result in renal hypoperfusion that, if left untreated, may lead to acute kidney failure. Some populations, notably older people and neonates, are less tolerant of extremes in fluid loading and deprivation, similar to those with established chronic kidney disease. Risk of kidney injury during fluid depletion is increased by medications including diuretics, nonsteroidal antiinflammatory drugs, and renin-angiotensin system blockers. There is no consistent evidence indicating that lower-than-average fluid intake can cause chronic kidney disease, nor accelerate progression of established kidney disease. Increasing consumption of sugar-containing beverages is, however, a major concern for kidney health as a precursor of obesity and diabetes. There is no evidence that high dietary protein intake can cause chronic kidney disease, nor accelerate progression of established kidney disease. Idiosyncratic, adverse renal responses have been described with creatine supplements. There are only a few clinical conditions for which high fluid intake should be considered. These include recurrent kidney stones or urinary tract infections and, possibly, polycystic kidney disease.