Corrosion tests have indicated that, except for pH, water composition within the range of variables studied has a minor influence on corrosion rates of carbon steel under conditions which may exist in the primary coolant of water-cooled and moderated reactors. Water conditions, however, cannot be considered unimportant, because (a) the range of variables studied has been limited by considerations of induced radioactivity and radiation stability of possible corrosion inhibitors, (b) special considerations associated with the primary coolant technology of reactor systems make water conditions important, and (c) the influence of water condition on the characteristics of the corrosion products formed may be of considerable concern. Some inhibitors were effective in reducing corrosion rates in autoclave tests but unsuitable for reactor operation. The possibility is discussed of using inhibitors during precritical operation of a new system to minimize the high corrosion rates normally experienced with new surfaces. The relationship existing between water condition and corrosion product behavior in the primary system of a water-cooled reactor is noted. Possible undesirable results of the adverse behavior of system corrosion products are: (1) fouling of the reactor core or other heat transfer surfaces by deposition, (2) fouling or wear of valves or mechanisms which operate in the primary system water, and (3) rapid pressure drop build-up on some types of filters used to remove the corrosion products. Current water technology studies have as their objective the operation of the primary systems of water-cooled and moderated reactors with the minimum control of water composition compatible with trouble-free operation. The establishment of minimum water-quality control requires knowledge concerning variations in water composition that may occur in reactor operation and how these variations affect plant integrity.