Breast cancer is the most frequent malignancy of women in the Western world. Vitamin D compounds constitute a novel alternative to the conventional use of antiestrogens for chemoprevention and chemotherapy. The biologically active form of vitamin D, 1,25-Dihydroxyvitamin D3 [1,25(OH)2D3], not only plays an essential role in the control of calcium homeostasis, but also acts on cells of a variety of tissues to promote inhibition of cellular proliferation and induction of differentiation. The potential use of 1,25(OH)2D3 in the treatment of cancer is limited by its propensity to cause hypercalcemia at pharmacologically active doses. This has led to the synthesis of analogs of vitamin D that exhibit potent anticancer effects, but have low calcemic activity. Evidence from both in vitro and in vivo studies has demonstrated that vitamin D compounds can inhibit the growth of breast cancer cells, suggesting their therapeutic value in the treatment or prevention of this disease.