X-ray diffraction studies have shown that there are several different kinds of human urinary calculi, with different age, sex, period, and geographical distributions. Juvenile bladder stones are typically urate and oxalate in small boys in certain stone belts. They have disappeared in some areas, particularly in Britain, but are still common in Thailand. India. and Turkey. Their cause is unknown. Adult bladder stones, formerly common in elderly men, were largely of uric acid and were due to a faulty diet. Juvenile kidney stones are rare, except in Turkey where they are similar to juvenile bladder stones. Adult kidney stones are by far the most universally common, especially in technically developed communities. They are found in both sexes (equally at postmortem), and in the United States and in Czechoslovakia the average number of hospital entries for stones, relative to the whole population, is about 1 per 1000 per annum (increasing) although the incidence in different districts varies by 4 to 1 or more. Such stones are mainly calcium oxalates and calcium and MgNH(4) phosphates. The incidence among the administrative class is at least 20 times that among agricultural workers, relative to their numbers. Stones are reported also to be an occupational hazard for air pilots. It is probably that much more exercise and the drinking of more water to prevent kidney dehydration (spirits and coffee are not effective for this purpose) would lower the high rate of incidence. Moderate acidification would prevent phosphate supersaturation of the urine, but is not effective for oxalates. It seems certain that, once a suitable seed is formed, epitaxy is largely responsible for deposition from urines that would otherwise remain supersaturated until voided. This would explain the curioLls radial and layered texture of many stones. Laboratory experiments might suggest ways of preventing orientated overgrowth.