This chapter addresses the role played by influences during intrauterine or early postnatal life in establishing the risk of osteoporosis in later years. At any age, the amount and quality of an individual's skeleton reflect their experiences from intrauterine life through the years of growth into young adulthood. Epidemiological evidence that the risk of osteoporosis might be modified by the intrauterine and early postnatal environment has emerged from two groups of studies. First, the retrospective cohort studies in which bone mineral measurements were undertaken. Second, mother-offspring cohorts relating the nutrition, body build and lifestyle of pregnant women to the bone mass of their offspring. The two most-studied forms of epigenetic marking are DNA methylation and histone modification. The key nutrients likely to influence fetal bone development include calcium and vitamin D, and therefore this axis provides a model for investigating the epigenetic regulation of bone mass.