Pre-clinical and clinical studies increasingly suggest that the most potent nitrogen-containing bisphosphonates have antitumour effects. Bisphosphonates inhibit osteoclast-mediated bone resorption, and can thus delay the spread of skeletal metastases. Bisphosphonates might also inhibit tumour growth outside the skeleton by inducing apoptosis and inhibiting proliferation, adhesion, invasion, and angiogenesis of tumour cells. Recent trials suggest a possible synergistic antitumour effect between bisphosphonates and chemotherapy. A mechanism of action for bisphosphonates that could explain their possible antitumour effect and their synergy with chemotherapy is speculative and demands further investigation. The role of bisphosphonates in primary neoadjuvant treatment of breast cancer is promising, but must be further investigated.