Vitamin D, known for regulating calcium and promoting skeletal health, also plays a complex role in immune modulation. This chapter explores vitamin D’s immune functions, relevance to autoimmune disorders, infection susceptibility, and potential benefits of dietary supplementation. Vitamin D receptors on immune cells facilitate active metabolite formation, highlighting its dual role in bone homeostasis and immune regulation. Low blood levels are linked to higher autoimmune risk. While supplementation shows promise, clinical results are inconclusive, requiring further research. Optimal vitamin D levels (40–60 ng/mL) via sun exposure and supplements are recommended. © 2025 Elsevier B.V., All rights reserved.