The quality of water is more and more important in the polluted world of today. The nephrologist is with his patients a great consumer of water; water for dialysis should be of the highest quality as each maintenance dialysis patient uses up to 150 liters 3 times per week for years, in direct contact with his blood through a very thin dialysis membrane. Water may be polluted at the collection-, treatment- or distribution centers or locally at the place of consumption. Aluminum, chloramine, pesticides, copper, herbicides and many other substances as well as bacteria, may be found in water. For these reasons the nephrologist uses a series of devices to clean or purify the water before use in his dialysis unit: softeners, de-ionisators, reverse-osmosis, charcoal filters, ultraviolet irradiation, a.o. A correct water circuit is important as well, as dead spaces could cause bacterial contamination. The ultra pure water has to be mixed before use in the dialyzer, with a concentrated electrolyte solution. The author discusses all these different aspects of water treatment in the particular situation of hemodialysis.