Epidemiological studies have demonstrated that vitamin D status is inversely associated with breast cancer incidence, mortality, and recurrences, suggesting vitamin D as a potent agent to reduce the risk of breast cancer. The hormonally active metabolite of vitamin D, $1{alpha},25$-dihydroxyvitamin D ($1{alpha},25(OH)_2D$, calcitriol), and its analogs have exerted inhibitory activity of cellular proliferation through arresting cell cycle and inducing apoptosis, and suppressive effects on the invasion, angiogenesis, and metastasis of breast cancer. In the studies of molecular basis of vitamin D activities, many upstream signaling pathways cross-talking with vitamin D signaling have been investigated. $1{alpha},25(OH)_2D$ and its analogs regulates different signaling pathways mediated by transforming growth factor-${eta}$ superfamily, epidermal growth factor receptors family, estrogen signaling-related molecules, insulin-like growth factor-binding proteins, and protein kinase C. The multipotent activities of vitamin D in signaling modulation may be efficient and effective in suppressing highly heterogeneous breast cancer.