Vitamin D deficiency is a global health problem. Vitamin D has attracted attention since evidence emerged that it may have functions beyond the control of serum calcium, for example immune system functioning and protection against damage to blood vessels. Gaps in the evidence base relating to vitamin D are also acknowledged. In this article, the production and functions of vitamin D are described, and the effects of vitamin D deficiency on pregnant women, infants, older people and other at-risk individuals are outlined. The management of vitamin D deficiency and the benefits of vitamin D supplementation are also identified.