Vitamin D is the third steroid hormone playing important biological roles in the development of breast cancer. Decreased plasma levels of its 25- hydroxyderivative, 25-OHD, display robust associations with higher incidence of breast cancer and shorter overall survival. Although no consensus exists, most authors agree that optimal plasma levels shall be within 75– 150 nmol/ l whereas levels higher than 375 nmol/ l can be potentially toxic with higher risk of hypercalcemia. To date, no data are available on the optimal levels of vitamin D related to the risk of breast cancer development, its phenotype features and the course of the disease. Published studies mostly describe associations among higher levels of 25-OHD and lower biologically aggressiveness of the tumor. The polymorphism of VDR gene coding for the steroid receptor for vitamin D may be associated with higher disease incidence and also be of negative prognostic signifi - cance in breast cancer. This review presents an overall summary of the current knowledge and publications on vitamin D and breast cancer.