Vitamin D belongs to a group of fat-soluble secosteroids which assume many roles in the human organism. In humans the most important forms are vitamin D3 and vitamin D2. Their primary function is the regulation of the calcium and phosphorus balance, which promote the growth of healthy bony tissue. Studies over the past few years have revealed a much wider role of vitamin D involving the aging processes, carcinogenesis, the carbohydrate balance as well as the effects on the course of various infections. In this paper we discuss the basic functions of vitamin D in the human body and the mechanisms of its activity and we summarize recent reports on the impact of vitamin D on the oral cavity with a special emphasis on autoimmunologic diseases, including: recurrent aphthous stomatitis, Behçet syndrome and Sjögren syndrome.