The advent of genetically engineered mouse models (GEMs) of human breast cancer, have provided important insight into molecular basis or human breast cancer. This review will focus on two of the most extensively studied mouse models for human breast cancer involving mammary gland specific expression of the polyoma middle T (PyV MT) antigen and of the ErbB2. In addition, this review will discuss past and recent advances in understanding relative contribution of the signaling pathways in tumor induction and metastasis by these potent mammary oncogenes.