Osteoporosis is a very common disorder affecting millions of post-menopausal women and men of various ages. Although the disease is manifested by painful fractures of the spine, hip or radius, the underlying pathogenesis is complex and multifactorial. One of the strongest predictors of future osteoporotic fractures is low bone mineral density. The determinants of adult bone density include the rate of bone acquisition during adolescence and the absolute loss of bone during the six decades of adult life. Recent studies have clarified how bone mass is acquired during the early teen years in both boys and girls. Genetic factors play an enormous role in defining the height of acquisition of bone mass; however, these factors also interact with environmental and hormonal determinants. Many more studies have focused on adult bone loss. Disorders in bone remodelling result in an imbalance in bone turnover, favouring resorption over formation. Systemic factors such as oestrogen deprivation and parathyroid hormone strongly activate remodelling and can, in several circumstances, lead to imbalances in the remodelling cycle. The molecular cues that couple bone formation to resorption have recently been elucidated, and those factors may themselves become therapeutic targets for future treatment regimens to prevent osteoporosis and its resultant fractures.