The general changes in calcium metabolism of laying birds all appear to be related to the formation of an egg with a well-calcified egg shell. The outstanding peculiarity of the avian reproductive system would be the extreme rate of calcium mobilization necessary for the formation of the egg shell. Although recent studies on this subject have revealed many valuable mechanisms by which calcification of the egg shell is controlled, there are some aspects which are remained for discussion. Much of the material considered in this article is related to the calcium metabolism for the formation of the egg shell in the fowl. Calcium and phosphorus retention generally rises over a period of 2 weeks before laying begins. This phenomenon should be based on the synergistic action of both sets of estrogens and androgens, since simultaneous administration of these hormones evokes an increase of calcium and phosphorus retention and the use of estrogens or androgens alone does not appreciably affect the changes in blood calcium and phosphorus in immature birds. Evidence is presented to show that much calcium is stored during reproduction in a special system of bone in the skeleton. The significance of hormones other than estrogens and androgens is discussed. The weight of evidence suggests that both medullary bone and cortical bone participate in shell formation. The structure of the avian egg shell is briefly described and its formation is related to the changes which occur in the calcium metabolism of the fowl. The significance and mode of action of the carbonate and calcium ions in the blood, uterine mucosa and uterine fluid during shell formation are discussed. Blood acid-base balance parameter is solely involved in shell formation. Metabolic acidosis, ventilatory alcalosis and acute hypercapnia may have a detrimental effect on shell formation due to a pH drop or a loss of bicarbonate. It is also discussed that the detrimental effect of carbonic anhydrase inhibitors on shell formation can be attributed to its powerful salidiuretic which disturbs the acid-base valance in the blood. Derangement of certain physiological mechanisms of the ovary and oviduct, which are unlikely to be related to calcium metabolism, can lead to the production of softshelled eggs. A mechanism of shell formation linked with ovulation cycle is briefly discussed.