Water is considered an essential nutrient, but very often is overlooked in favour of nutrition. Due to age related changes, older people are more vulnerable to both, dehydration and overhydration. This article describes the principles of water homeostasis and provides a description of common age-related physiological changes related to water balance. Fluid homeostasis largely depends on osmo- and baro-receptors that respectively react to changes in osmolality and blood pressure, water-regulating hormone release, the subsequent response of kidneys to the hormonal stimulation and activation of thirst. These mechanisms diminish with age and put elderly at risk of dehydration. All involved in a care for the elderly should be informed and become vigilant to prevent dehydration from occurring.