Studies performed in experimental animal models have demonstrated that mammary cancer is a complex multistep process that can be induced either by chemicals, radiation, viruses, or genetic factors. Rodent models have been useful for dissecting the initiation, promotion, and progression steps of mammary carcinogenesis. Chemically induced mammary tumors, such as those induced by 7,12-dimethylbenz(a)anthracene and N-methyl-N-nitrosourea, are, in general, hormone-dependent adenocarcinomas whose incidence, number of tumors per animal, tumor latency, and tumor type are influenced by the age, reproductive history, and endocrinologic milieu of the host at the time of carcinogen exposure as well as by diet and the dose of carcinogen administered. There is a need to classify tumors according to their histopathological type because those characteristics have implications in the interpretation of experimental data. In the classification presented here we attempt to provide a working framework for diagnosis of the type of lesions found in the mammary glands of rats treated with chemical carcinogens or radiation and to clarify criteria for establishing the basic biological characteristics of tumors.