Providing enough water of good quality to all human communities is a difficult task, which has been satisfied only recently and only for the developed world. A large part of the developing world still suffers from scarcity and/or bad quality of water supply. Examples from the past are described, including the cholera epidemics of London 1848-1853 and the chromium pollution of the Milan area, 1958. A synthetic description of the different kinds of biological and chemical pollution are also described, then the complex mechanisms of biological and chemical pollution of the waters are illustrated, which require complicated interventions for reclamation after pollution or, better, even more complicated surveillance to avoid pollution. Finally the problem of safeguard of waters during the distribution is illustrated, when a bad maintenance of the aqueducts can inactivate all the precautions taken during the supply an the treatment of waters.