Vitamin D has long been known to be important for bone health, but there is currently no consensus on the amount of vitamin D needed or how it varies with age, race/ethnicity, body size, absorption efficiency, season, and other factors. This review describes the effects of vitamin D on calcium absorption, parathyroid hormone levels, and changes in bone mineral density briefly and focuses in more detail on the results and interpretation of double-blind randomized controlled trials with fracture outcomes in older adults. Based on these trials, 400 IU/day, bringing 25-hydroxyvitamin D (25OHD) levels to 60-65 nmol/L, is not effective at lowering fracture risk in community-dwelling elders or in elders with prior fractures. Several large trials indicate that doses in the range of 700-1,000 IU/day can lower fracture risk by ~20 %. From these trials, it appears that a 25OHD level of 65 nmol/L is needed to reduce nonvertebral fracture risk and 75 nmol/L may be needed to lower hip fracture risk. Trials testing additional doses of vitamin D in different segments of the population are needed.