Epithelial-mesenchymal-transition (EMT) tumorigenesis in the mouse was first described over 100 years ago using various terms such as carcinosarcoma and without any comprehension of the underlying mechanisms. Such tumors have been considered artifacts of transplantation and of tissue culture. Recently, EMT tumors have been recognized in mammary glands of genetically engineered mice. This review provides a historical perspective leading to the current status in the context of some of the key molecular biology. The biology of mouse mammary EMT tumorigenesis is discussed with comparisons to human breast cancer.