Diet is considered a key lifestyle factor in influencing breast cancer risk, and thus a promising tool to attempt to prevent this disease. However, the exact food groups we should consume and those we should avoid to prevent breast cancer continue to remain unclear. Focusing on the 3rd Expert Report by the WCRF/AICR, in this chapter we discuss the current knowledge about limiting fat intake to 25% of total energy intake, and consuming plenty of fruits and vegetables, especially those that contain β-carotenoids. No specific fatty acid (saturated or unsaturated) seems to be better or worse than any other. We also will provide evidence in support of incorporating legumes high in isoflavone genistein to the diet to prevent breast cancer and its recurrence, and after breast cancer diagnosis, supplementing diet with vitamin D if a patient’s vitamin D levels are not sufficient.