Calcitriol, or 1,25-dihydroxyvitamin D3 (1,25(OH)₂D3) is a well-known endocrine regulator of calcium homeostasis. More recently, local calcitriol production by immune cells was shown to exert autocrine or paracrine immunomodulating effects. Immune cells that produce calcitriol also express the vitamin D receptor (VDR) and the enzymes needed to metabolize vitamin D3 (1⍺-, 25-, and 24-hydroxylases). Studies of animal models and cell cultures showed both direct and indirect immunomodulating effects involving the T cells, B cells, and antigen-presenting cells (dendritic cells and macrophages) and affecting both innate and adaptive immune responses. The overall effect is a switch from the Th1/Th17 response to the Th2/Treg profile. The immunomodulating effects of vitamin D may explain the reported epidemiological associations between vitamin D status and a large number of autoimmune and inflammatory diseases. Such associations have been suggested by observational studies not only in rheumatoid arthritis, lupus, inflammatory bowel disease, and type 1 diabetes; but also in infections, malignancies, transplant rejection, and cardiovascular disease. In animal models for these diseases, vitamin D supplementation has been found to produce therapeutic effects. Thus, vitamin D is a key focus for public health efforts and may hold promise for the treatment of dysimmune diseases.