In recent years, there has been great interest in the role of vitamin D in a number of diverse human diseases including autoimmunity, allergy, infection, cardiovascular disease, chronic lung disease, transplantation and cancer. Vitamin D is best known for its role in calcium metabolism; however it also has potent immunomodulatory effects. Epidemiological studies suggest that vitamin D deficiency may be a significant risk factor for many diseases. Furthermore, there is accumulating evidence from experimental studies that vitamin D has anti-inflammatory effects. Recent studies have indicated that a surprisingly high proportion of people are vitamin D deficient, suggesting that vitamin D supplementation may be of benefit to human health. This review will focus on the role of vitamin D in autoimmune diseases, including multiple sclerosis, rheumatoid arthritis and diabetes. We will review the epidemiological and experimental evidence for the protective effects of vitamin D in autoimmunity, as well as the preliminary vitamin D intervention studies and the most recent patented vitamin D analogues.