Vitamin D plays a key role in bone metabolism. The link between vitamin D deficiency and rickets is well understood. However, subclinical vitamin D deficiency may also be detrimental to bone health in childhood. Its effects on bone mineralization have the potential to result in lower peak bone mass being attained, which could in turn contribute to increased fracture risk in both childhood and older adult life. As vitamin D deficiency is common globally, any detrimental effects of vitamin D deficiency on bone health are likely to have substantial public health implications. This review describes the current literature relevant to vitamin D and bone health in childhood and adolescence, with a particular emphasis on evaluating the emerging evidence for the impact of subclinical vitamin D deficiency on bone health and the effectiveness of vitamin D supplementation. The evidence suggests that subclinical vitamin D deficiency does affect bone acquisition, potentially beginning in utero and extending into adolescence. However, the effectiveness of vitamin D supplementation for improving bone health in situations of subclinical deficiency remains unclear, particularly in early life where there are few trials with bone density outcomes. The available evidence suggests that benefits are likely to be greatest in or even restricted to children with serum 25-hydroxyvitamin D levels at least below 50 nmol/L and possibly even lower than this. Trials of sufficient duration in deficient pregnant mothers, infants, and children are urgently required to address critical evidence gaps.