The secosteroid hormone 1alpha,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) is the natural ligand for the vitamin D receptor, a member of the nuclear receptor superfamily. Upon binding of the ligand, the vitamin D receptor heterodimerizes with the retinoid X receptor and binds to vitamin D response elements in the promoter region of target genes to induce/repress their expression. The target genes that have been identified so far are heterogeneous in nature and reflect the great spectrum of biological activities of 1,25(OH)(2)D(3). Within the last two decades, the receptor has been shown to be present not only in classical target tissues such as bone, kidney, and intestine, but also in many other nonclassical tissues, for example, in the immune system (T and B cells, macrophages, and monocytes), in the reproductive system (uterus, testis, ovary, prostate, placenta, and mammary glands), in the endocrine system (pancreas, pituitary, thyroid, and adrenal cortex), in muscles (skeletal, smooth, and heart muscles), and in brain, skin, and liver. Besides the almost universal presence of vitamin D receptors, different cell types (for example, keratinocytes, monocytes, bone, placenta) are capable of metabolizing 25-hydroxyvitamin D(3) to 1,25(OH)(2)D(3) by the enzyme 25(OH)D(3)-1alpha-hydroxylase, encoded by CYP27B1. The combined presence of CYP27B1 and the specific receptor in several tissues introduced the idea of a paracrine/autocrine role for 1,25(OH)(2)D(3). Moreover, it has been demonstrated that 1,25(OH)(2)D(3) can induce differentiation and inhibit proliferation of normal and malignant cells. Moreover, vitamin D deficiency is associated with an increased risk for nearly all major human diseases such as cancer, autoimmune diseases, cardiovascular, and metabolic diseases. In addition to the treatment of bone disorders with 1,25(OH)(2)D(3), these newly discovered functions open perspectives for the use of 1,25(OH)(2)D(3) as an immune modulator (for example, for the treatment of autoimmune diseases or prevention of graft rejection), inhibitor of cell proliferation, and inducer of cell differentiation (cancer).