**1. Introduction**

Vitamin D is a secosteroid hormone, which is known to be related to the regulation of the musculoskeletal system. It affects calcium and phosphate metabolism and is related to bone health. Recently, the extraskeletal effects of vitamin D are under intense research and have attracted the interest of the scientific community [1–6]. In particular, the relationship of vitamin D with the immune system is in the focus of scientific evaluation [7–9]. In the chapter herein, the effects of vitamin D on the immune system will be discussed, and the relationship of vitamin D deficiency with the development of autoimmune diseases will be reviewed.
