Water supply is a basic public problem. In modern science, three periods with different approaches to define recommended water intake in adults can be distinguished. Pediatricians agree that hydration in children may be optimal only in breastfed infants. More data are required on the health effects of different hydration states and varying water intakes in particular age and gender groups to define optimal ranges of water intake. The fetus grows in an exceptionally well-hydrated environment. Water metabolism shows several peculiarities in preterm and term infants. Infant diarrhea remains a major topic of basic and clinical research. Water intoxication in infants, toddlers, and children is rare and can only be found in exceptional circumstances. Hydration status characterized by hyponatremia may play a role in the pathogenesis of febrile convulsions in toddlers. There is increasing indirect evidence that spontaneous drinking behavior of a population may be fixed and anchored in the age range of toddlers. Sex differences in hydration status are common, but not obligatory. What causes theses differences? What is behind the various circadian rhythms of urine osmolality in children? At what age and in what quantities can alcohol and caffeine consumption be tolerated? How can individual susceptibility be defined? Reflecting on the modern epidemic of obesity in children and adolescents, a public consensus concerning use and misuse of sweetened drinks seems mandatory. Dietary reference intakes of water refer to 24-hour intake. In nutritional counselling, food and meal-based dietary advice is primarily given. Young parents are confronted with a flood of advice of varying quality. Recommendations on fluid consumption should be collated and revised.