Widespread vitamin D insufficiency and vitamin D supplementation (even low dose rickets prophylaxis) have been hypothesised as contributory factors to the recent increase in asthma. These hypotheses are supported by reports of immunomodulatory effects of vitamin D on antigen presenting cells, regulatory T cells and T-helper cells and evidence that vitamin D influences fetal lung differentiation and epithelial-mesenchymal function. Studies of vitamin D in animal models confirm complex effects of vitamin D on asthma immunopathogenesis. In humans a majority of epidemiological studies support the hypothesis that vitamin D insufficiency during pregnancy increases the likelihood of childhood wheeze and possibly asthma, although some studies do report the converse. In children and adults with asthma, reduced serum vitamin 25-hydroxyvitamin D levels have been associated with parameters of increased asthma severity. Clinical trials are underway addressing whether maternal vitamin D supplementation during pregnancy reduces the likelihood of childhood asthma and if there is a role for vitamin D supplementation in established asthma.