Purpose of Review: The purpose of this chapter is to describe physiological changes occurring with age relevant to fluid therapy, and to discuss the influence of fluid therapy on outcome of surgery from this perspective. Recent Findings: Dehydration, common among elderly people, is often mistaken for dementia or depression. Dehydration with dizziness increases the risk of falling, a predictor for adverse outcomes after surgery. Young volunteers can excrete up to 300 mmol sodium per liter urine. Elderly people cannot achieve such a concentration emphasizing the importance of giving water to excrete the osmoles given with intravenous crystalloids. Brain natriuretic peptide increases in cardiac disease and with age. Preoperative elevated levels predict adverse outcomes following surgery. Summary: The evidence and the physiology show that elderly patients benefit from a thorough regulation of fluid therapy keeping fluid balance close to zero (no more than 2 l positive) and with sufficient glucose and water supply.