Advances in biotechnology and basic immunology have converged to create an unprecedented opportunity to use vaccines to harness the power of the immune system in the fight against breast cancer. Cancer vaccines have several therapeutic advantages over more traditional breast cancer treatment modalities. First, targeting the antitumor immune response to critical tumor-specific antigens defines a therapy with exquisite specificity and minimal toxicity. Second, immune-mediated tumor destruction occurs by mechanisms distinct from those underlying the efficacy of chemotherapy and hormone therapy. Thus, immunotherapy offers an approach to circumventing the intrinsic drug resistance that currently underlies therapeutic failure. Third, the phenomenon of immunologic memory endows immunotherapy with the potential for creating a durable therapeutic effect that is reactivated at the onset of disease relapse. Moreover, immunologic memory also underlies the potential future use of vaccines for the prevention of breast cancer. Early clinical trials have highlighted the promise of breast cancer vaccines, and have further defined the challenges facing translational scientists and clinical investigators. The judicious application of laboratory advances to clinical trial design should facilitate the development of immunotherapy as an additional major therapeutic modality for breast cancer, with the potential for breast cancer prevention as well as treatment.