Efflorescence, the unsightly but harmless powdery coating of chemical salts that occasionally appears on concrete and masonry surfaces, is easily prevented by good construction practices. Fortunately, where deposits do appear, they can be easily removed. Two conditions are necessary for the occurrence of efflorescence: (1) a source of soluble salt, and (2) a transportation system to bring these chemical salts to the concrete surface. Prevention, therefore, consists of both reducing the concentration system by which moisture containing the chemical salts moves through the concrete. The concentration of salts in the cement batch can be reduced by using only clean aggregates and fresh water free of impurities. The transportation system can be blocked by using well-graded aggregates, adequate cement content, and low water/cement ratios in the batch. In addition, masonry joints should be formed from mortars of suitably graded materials, well-bedded and tooled to give complete weather-tightness. Also, since the soil over which concrete is placed may contain salts, a waterproof membrane between the soil and concrete is required to prevent the upward migration through the concrete of ground water containing chemical salts. At, times, however, when good concreting procedures have not been followed, the concrete may absorb rainwater that will later return to the surface and deposit materials on evaporating, should efflorescence occur under these conditions, it may be controlled by protecting the weather face of the concrete with a surface sealant. If a sealant is used, the work should be done during a period of dry weather when the moisture content of the concrete is likely to be low. Removing efflorescence when it does appear is relatively simple. Many deposits can be removed with a stiff bristled brush, especially if brushed soon after deposits appear. Since most salts in the concrete are soluble in fresh water, hosing, perhaps at the time of brushing, should be tried. Efflorescence that cannot be removed by these simple methods may require sandblasting or washing with acid.