The vitamin D endocrine system is critical for the maintenance of circulating calcium concentrations, but recently, there has been advocacy for the widespread use of vitamin D supplements to improve skeletal and nonskeletal health. Recent studies of tissue-selective vitamin D receptor knockout mice indicate that the principal action of vitamin D responsible for the maintenance of calcium homoeostasis is the regulation of intestinal calcium absorption. High levels of vitamin D can increase bone resorption and impair mineralization, consistent with its role in maintaining circulating calcium concentrations. These findings suggest that circumspection is appropriate in its clinical use. There is now substantial clinical trial data with vitamin D supplements, which fails to establish their efficacy on bone density or the prevention of falls or fractures. However, some trials in frail and/or vitamin D-deficient populations have produced positive outcomes. Where there are positive effects of vitamin D supplementation on skeletal outcomes, these are mainly seen in cohorts with baseline circulating 25-hydroxyvitamin D (25(OH)D) levels in the range 25-40 nmol/L or lower. A great diversity of nonskeletal conditions have been associated with low 25(OH)D, but there is little evidence for efficacy of vitamin D supplementation for such end-points. At present, supplements should be advised for populations with risk factors (e.g., lifestyle, skin color, and frailty) for having serum 25(OH)D levels in the 25- to 40-nmol/L range or below. A dose of ≤800 IU/day is adequate. This approach will maintain 25(OH)D levels well above the threshold for osteomalacia and makes allowance for the poor accuracy and precision of some 25(OH)D assays.