Elements of the immune system act as intimate regulators of cancer progression, inhibiting early stages of tumor growth, through immunosurveillance while facilitating later stages of tumor progression. Recent findings have revealed that activated CD8 T cells can stimulate mammary epithelial tumor cells to undergo epithelial-mesenchymal transition (EMT) and to acquire the greatly increased tumorigenic capability and chemotherapeutic resistance of breast cancer stem cells (BCSC). These studies provide a window to understanding how BCSC arise and are maintained within tumors, and how to best target these processes for therapeutic benefit.