Atopic dermatitis (AD) is a highly prevalent allergic skin disease that affects children worldwide. Epidemiological, clinical and basic immunological studies have suggested an association between vitamin D (VD) deficiency and the development of AD and other allergic diseases. Low levels of VD, a pleiotropic hormone that has widespread effects on the immune system and skin integrity, appear to be inversely correlated with AD severity, and VD deficiency at birth is associated with higher risk of developing AD. Recent randomized trials have reported inconsistent, but still promising, results on whether VD supplementation may improve AD severity. The apparent differences are likely due to small samples, different regimens and different patient populations. The mechanisms underlying this potential improvement in AD severity remain under investigation.