Osteoporosis is the net result of the maximal amount of bone mineral mass achieved by the end of pubertal growth (peak bone mass) minus post-menopausal and elderly bone losses. Peak bone mineral mass is determined from early childhood by both heritable and environmental factors. Recent developments in the molecular epidemiology of osteoporosis have shown the interest, but also the limitations, of specific molecular markers, such as the vitamin D receptor gene polymorphisms Bsm 1 and Fok 1, to explain bone mineral density differences across the population. Importantly, however, interactions between VDR gene polymorphisms and environmental factors, particularly dietary calcium, have provided new insights into the complex determination of bone mineral mass.