There has been a continuous effort to understand possible non-Ca metabolism roles of vitamin D, including its role in the immune system and, in particular, in T cell-medicated immunity. Vitamin D receptor is found in significant concentrations in the T lymphocyte and macrophage populations, when we refer to immune system, and pretty much in any human tissue and cells. Until the eighties, no one had imagined that vitamin D might play a role in the functioning of the immune system. Today we accepted that the normal immune system harbors a regulatory T cell (Treg) population specialized for immune suppression. Currently, the most commonly known regulatory T-cell lineage is called CD4+ CD25high FoxP3+ regulatory T cells. Several autoimmune disorders have been linked to a deficiency in vitamin D3. In some autoimmune diseases, including multiple sclerosis (MS), a compromised Treg function is believed to be critically involved in the disease process. Vitamin D insufficiency has ramifications not only for bone health, but also in other non-skeletal areas of vitamin D function, such as immune cells, muscle cells and, perhaps, adipocytes. As a final conclusion, further researches in the field of vitamin D, Tregs, immunity (inflammatory processes, rejection, autoimmune diseases, etc.), either in vitro on cell cultures or in vivo using lab animals or volunteers are still necessary.