Vitamin D should not be considered only as a vitamin. It has a relevant role in many functions of body regulation, both skeletal and extra skeletal and this makes vitamin D an essential element for a healthy status. This is well explained by a ubiquitous presence of vitamin D receptors. Nowadays extra skeletal effects have a more interesting impact in medical practice. The paracrine and autocrine action of vitamin D has a pivotal role for these effects. The activation of the cellular transcriptional process leads to the expression of beta-defensin and cathelicidin, activating the Th1 pathway, related to innate immunity against bacteria. The action of vitamin D is also related to adaptive immunity with a Th2 response and production of anti-inflammatory cytokines like interleukins 4 and 5, and with Th17 and B-lymphocyte suppression. Vitamin D deficiency could have an unfavorable effect on both healthy and ill subjects. It is well-known that many autoimmune diseases like systemic lupus erythematosus and rheumatoid arthritis are influenced by vitamin D deficiency, and this is especially true for disease activity. Several other pathologies are influenced by the levels of vitamin D, such as diabetes mellitus type 1: an adequate intake of vitamin D can reduce the risk to develop this disease. The same applies to asthma and multiple sclerosis. It is very important to make a point about the deficiency state and their correction, especially in those people at higher risk.