“A knowledge of the factors affecting the precipitation and solution of calcium carbonate? and calcium phosphate underlies the solutions of such physiological problems as bone and tooth formation and such pathological problems as rickets and arteriosclerosis.” l This statement gives a clear directive to turn toward the relatively simple physicochemical problems in the calcium-phosphate-water system to gain some understanding of the phenomena of precipitation and solubility. In the prescnt brief review, it is our intention not to consider the multiple complications unavoidably present in the biological system, and to forget, for the moment, that vitamin D, various hormones, conditions of oxygenation, and degree of COz saturation, for example, all may play important roles in oizio. Ultimately, the interaction of these factors must be all woven into the story of the growth, the health, and the diseases of bones and teeth. For a slart, however, even the comparatively simple process of the formation of hydroxylapatite in vitro must be described as a mystery. The solution process also is so complicated that only recently reproducible values have been obtained.