Osteoporosis is a major health problem worldwide, particularly with advancing age in both men and women. The strength of the skeleton in older age results from bone strength achieved in early adulthood and age-related and, in women, post-menopause-related bone loss. While trauma and the manner in which older people fall are important contributors to fracture risk, low bone mass is a major factor. Determinants of bone mass include external factors such as lifestyle, especially physical activity, and calcium intake. The wide variation in dietary calcium intake across countries does not correlate with osteoporotic fracture risk, presumably due to ethnic differences between and within populations. The twin approach has been useful in the identification of the major part of age-specific variation in bone density (and turnover), which is genetically determined. Exploring possible genetic factors, we reported that common allelic variations in the vitamin D receptor (VDR) gene were associated with indices of bone turnover and density. Subsequent studies, including our own, have found weaker effects. However, allelic effects of the VDR gene polymorphisms have now been reported in a wide range of, but not all, Caucasian and Asian populations in which they have been studied. In relation to possible physiological mechanisms, the VDR alleles correlate with differences in gut calcium absorption and response of bone density to long-term dietary calcium intake. Moreover, differences in response to active vitamin D compounds have been found in relation to VDR gene alleles. Understanding how these allelic variants, which are not associated with differences in