The skeleton is the most common site of breast cancer metastases. These bone metastases are usually osteolytic and cause significant morbidity. Bisphosphonates, potent inhibitors of bone resorption, reduce skeletal morbidity in breast cancer patients with bone metastases. Animal studies with bisphosphonates are crucial to understanding the mechanisms by which these compounds affect bone and tumor cells in vivo. Such animal models of breast cancer that are used to test the efficacy of bisphosphonates are discussed. These studies may offer insight into the treatment of other tumor types that frequently metastasize to bone.