The active metabolite of vitamin D3, 1,25-dihydroxyvitamin D3 (1,25(OH)D3), has been recognized for over 2 decades as a modulator of cell proliferation and differentiation in many cell types, including breast cancer. However, any potential anti-tumour properties displayed by 1,25(OH)D3 are limited by the tendency to cause hypercalcaemia when administered at high doses. Because of this, synthetic vitamin D analogues have been developed that retain the anti-tumour effects seen with 1,25(OH)D3 but which have reduced calcaemic activity. However, it is still unclear as to how 1,25(OH)D3 and its synthetic analogues act within breast cancer cells to elicit the effects on cellular proliferation and differentiation. In this chapter we review the advances that have been made in trying to answer this question. It has been found so far that 1,25(OH)D3 has an effect on the expression of certain cell cycle regulators and in this way can bring about G1 arrest. Evidence has also emerged that vitamin D compounds can also affect the growth-promoting pathways initiated by two important factors involved in breast cancer cell promotion; namely the insulin-like growth factor I (IGF-I) and oestrogen-receptor (ER) pathways. Vitamin D compounds have also been implicated in promotion of apoptosis in breast cancer cells and evidence suggests that 1,25(OH)D3 and its synthetic analogues may potentiate responsiveness of breast cancer cells to conventional cytotoxic agents. Although much remains to be learned about the associated underlying mechanisms, ongoing research suggests that vitamin D analogues are a new class of compounds with potential in breast cancer treatment and prevention.