Vitamin D was identified nearly a century ago as an important regulator in the absorption and transport of essential minerals (particularly calcium, phosphorus and magnesium), and as such is a key determinant of normal bone development. Vitamin D is now seen as a potentially key hormonal regulator in the cardiovascular, musculoskeletal, renal, nervous, adipose and immune systems. An adequate supply of vitamin D at all stages of life is essential for the maintenance of good health. It has, however, been estimated that upwards of 1 billion people, in both the developed and developing world, can be categorised as being vitamin D deficient. This article discusses some of the key aspects of the biochemistry, physiology and pharmacology of vitamin D.