As the first mammal to be domesticated for research purposes, rats served as the primary animal model for various branches of biomedical research, including breast cancer studies, up until the late 1990s and early 2000s. During this time, genetic engineering of mice, but not rats, became routine, and mice gradually supplanted rats as the preferred rodent model. But recent advances in creating genetically engineered rat models, especially with the assistance of CRISPR/Cas9 technology, have rekindled the significance of rats as a critical model in exploring various facets of breast cancer research. This is particularly pronounced in the study of the formation and progression of the estrogen receptor-positive subtype, which remains challenging to model in mice. In this chapter, we embark on a historical journey through the evolution of rat models in biomedical research and provide an overview of the general and histological characteristics of rat mammary glands. Next, we critically review major rat models for breast cancer research, including those induced by carcinogens, hormones, radiation, germline transgenes, germline knockouts, and intraductal injection of retrovirus/lentivirus to deliver oncogenic drivers into mature mammary glands. We also discuss the advances in building rat models using somatic genome editing powered by CRISPR/Cas9. This chapter concludes with our forward-looking perspective on future applications of advanced rat models in critical areas of breast cancer research that have continued to challenge the mouse model community.