The immune system is active in breast cancer, playing a dual role in tumor progression and in immune surveillance. Infiltrating immune cells are both prognostic and predictive of response to standard breast cancer therapies. Breast cancer vaccines can activate and expand tumor-specific T cells, but have enjoyed minimal clinical success to date. Immune checkpoint blockade is a new approach to cancer immunotherapy, with documented clinical responses in diverse tumor types. Interest in breast cancer immunotherapy has been reignited by recent reports of objective responses in metastatic triple-negative breast cancer with both pembrolizumab (a programmed cell death protein 1 [PD-1] antagonist) and MPDL3280A (a programmed cell death ligand 1 [PD-L1] antagonist). Rational strategies for combination immunotherapy that expand and promote the trafficking of tumor-specific T cells, support their activity at the tumor site, and abrogate pathways of immune suppression within breast tumors are most likely to result in objective responses that translate into long-term disease control and cure.