Background: The effects of Vitamin D on bone metabolism and calcium homeostasis have already been well recognized. Emerging evidence has implicated Vitamin D as a critical regulator of immunity, playing a role in both the innate and cell-mediated immune systems. Emerging evidence has implicated Vitamin D as a critical regulator of immunity, playing a role in both the innate and cell-mediated immune systems. Vitamin D deficiency has been found to be associated with several immune-mediated diseases, susceptibility to infection and cancer. Further elucidation of the role of Vitamin D in lung development and immune system function may hold profound implications for the prevention and treatment of asthma.