Mammary gland cells from different species have an almost identical structure irrespective of the fact that the milk secreted by these cells varies widely in its chemical composition from one species to another. Differentiation of mammary gland begins in early embryonic development, but at birth mammary gland is poorly developed. The growth of the mammary gland continues during puberty and the fastest growth occurs during pregnancy. All stages of differentiation and development of the mammary gland and its later function are under complex endocrine control, in which various hormones are included. The most important between them are estrogen, progesterone, prolactine, growth hormone and insuline.like growth factor-I, but also a number of other hormones, including insulin, cortisol and aldosterone. In addition to being involved in the development of mammary gland, some of these hormones are a key factor in the carcinogenesis of that tissue. Many papers indicate on the involvement of estrogen and prolactin and their signal pathways in cell proliferation with already existing mutation or on increase opportunities for the occurrence of mutations, or in proliferation, survival and motility of cells and supporting vascularization of the mammary gland tumors. Nowadays, one of the major goals in human and veterinary medicine is to identify prognostic factors that could identify therapeutic targets to support a better outcome and to improve survival. Research on animals in vivo, and on organic cultures of mammary gland provided valuable information on various factors involved in the growth and differentiation of the mammary gland. Canine mammary tumors have been suggested as a model to study human breast cancer, for a number of years due to the great number of similarities between them, from epidemiological dana to the histological patterns of the neoplastic lesions. In addition, the high incidence of mammary tumors in dogs in relatively short period, because of their shorter lifetime, can be used as an advantage in carrying out clinical trials for earlier detection and more complete biochemical characterization of breast cancer.