Rodent models have been, and continue to be, important in both defining the etiology of breast cancer, and in generating new prevention and treatment strategies directed at this prevalent malignancy. Of all solid cancers, breast cancer has the widest variety of rodent models. Models in rats and mice include both spontaneous, induced and transplantable carcinomas in standard strains. In addition, many strains are available for modeling inherited susceptibility and resistance to breast cancer. Finally, many new models that have taken advantage of techniques from biotechnology such as transgenic, knock-out and gene therapy technologies are being used to evaluate new prevention and treatment strategies for breast cancer.