The effects of vitamin D on bone metabolism and calcium homeostasis have long been 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. Vitamin D deficiency has been found to be associated with several immune-mediated diseases, susceptibility to infection and cancer. Recently, there has been increasing interest in the possible link between vitamin D and asthma. 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.