This chapter seeks to outline the strategies being employed to exploit transgenic mice as a tool in the study of cancer. Rather than catalog the many different oncogene and oncogene-promoter combinations that have been introduced into transgenic mice to elicit tumors, I will concentrate on a single system that epitomizes the general approach. That system, pioneered by Philip Leder of Harvard University, is mammary carcinogenesis induced by oncogene expression directed by the Mouse Mammary Tumor Virus (MMTV) enhancer-promoter.