The evolution of water treatment control has been in response to the effect of water contaminants on observed damages in steam generators. The accumulation of deposits from condenser leakage constituents in combination with alkaline boiler water additives has caused corrosion in recirculating fossil boilers. This is mitigated by the use of phosphate treatment only at controlled pH conditions. The same fossil water technology is applied to nuclear boilers with adverse results, especially in units tubed with Alloy 600. The advent of once-through fossil steam generators led to the use of very pure water, since anything not soluble in the steam was available to concentrate and deposit in the boiler to enhance corrosion and heat transfer problems. This fact necessitated the introduction of condensate and feedwater polishing by filtration and ion exchange. When corrosion problems were encountered in nuclear steam generators due to phosphate chemicals in combination with condenser leakage constituents, pure water treatment philosophy was adopted, and with it condensate polishing came more widely into use in these units. Pure water with condensate polishing was always applied to once-through nuclear steam generators. Since no water treatment methods are 100% successful in controlling steam generator deposition and associated corrosion and thermal-hydraulic problems, chemical cleaning has been adopted as a maintenance procedure, first in fossil units and now in nuclear units