Vitamin D deficiency is highly prevalent and is increasingly thought to be an important risk factor in many diseases that have high morbidity and mortality, including lupus. Vitamin D is an immunomodulatory hormone with effects on T cells, B cells, and dendritic cells. Animal models of autoimmune disease and epidemiologic studies suggest a role for vitamin D as a modifiable environmental factor in autoimmune disease. Recommendations are available regarding screening for and repletion of vitamin D deficiency. More research is needed to understand the role of vitamin D as an immunomodulator and to determine the optimal range of serum 25-hydroxyvitamin D for musculoskeletal, cardiovascular, and immune health.