Mammary cancer occurs among all taxonomic groups, and comparing the disease in animals with breast cancer in women could greatly improve our understanding of the relevant risk factors and genetic profiles for this disease. Differences in cancer prevalence between carnivores and herbivores and between captive and wild carnivores are striking and support the hypotheses that diet and reproductive history are major risk factors. Domestic dogs and cats have a high prevalence of mammary tumors, and the majority of tumors in cats are aggressive cancers. Many domestic dogs and cats are prevented from breeding, resulting in their being exposed to recurrent estrogen peaks followed by high persistent levels of progesterone. Therefore progesterone appears to be a significant risk factor for cancer development. Supporting this suspicion is the observation that most mammary cancers in zoo cats are in those treated with the potent synthetic progestin contraceptive, melengestrol acetate. The more common morphologic types of mammary cancer in canids and felids include tubulopapillary, solid, cribriform, comedo and anaplastic carcinomas. Dogs also develop complex carcinomas, which likely evolve from the complex adenomas or mixed tumors that are so common in this species and are promoted by exogenous progesterone treatment. Among zoo felids, jaguars are at higher risk for mammary cancer and also have a high prevalence of ovarian papillarycystadenocarcinomas, a profile similar to women with BRCA1 mutations. As for women, estrogen (ER) and progesterone receptor (PR) expression varies in canine and feline mammary cancers. In general, ER expression is low, but PR expression persists in most cancers. Alterations in molecular controls of cell proliferation or survival in breast cancer, such as cyclin A and p53 expression, have been identified in dog and cat mammary cancers. Overall, spontaneous mammary cancers in cats and dogs make excellent models for human breast cancer, and knowledge of mammary carcinogenesis would be greatly enhanced across all species by a "One Medicine" approach.