In addition to its role in calcium homeostasis and bone health, vitamin D has also been reported to have anticancer activities against many cancer types, including breast cancer. The discovery that breast epithelial cells possess the same enzymatic system as the kidney, allowing local manufacture of active vitamin D from circulating precursors, makes the effect of vitamin D in breast cancer biologically plausible. Preclinical and ecologic studies have suggested a role for vitamin D in breast cancer prevention. Inverse associations have also been shown between serum 25-hydroxyvitamin D level (25(OH)D) and breast cancer development, risk for breast cancer recurrence, and mortality in women with early-stage breast cancer. Clinical trials of vitamin D supplementation, however, have yielded inconsistent results. Regardless of whether or not vitamin D helps prevent breast cancer or its recurrence, vitamin D deficiency in the U.S. population is very common, and the adverse impact on bone health, a particular concern for breast cancer survivors, makes it important to understand vitamin D physiology and to recognize and treat vitamin D deficiency. In this review, we discuss vitamin D metabolism and its mechanism of action. We summarize the current evidence of the relationship between vitamin D and breast cancer, highlight ongoing research in this area, and discuss optimal dosing of vitamin D for breast cancer prevention.