Among domestic animals, canines are those that most frequently suffer tumors of mammary gland and among them females are those that present the biggest number of cases. When these tumors are present in males, they have malign hystologic characteristics. For the hysto-pathologic diagnosis the best accepted classification is Hampe and Misford ' s proposal. Through many years of research in this field several predisposition factors, as race, sex and age have been described. Among these, race accounts for the highest disparity of concepts since, as some authors argue, this can be influenced by some external factors as geographic location and the owners ' likes for certain races as company animals. Another important fact is the description of causal agents such as hormonal imbalance (estrogens, progesterone), genetic mutations (suppressive gene of the tumor and proto- oncogenesis), consumption of fat-rich diets. Finally, it is important to emphasize the fact that the prognosis and life expectations of the canine patient depend on the basis of a good diagnosis and clues for treatment carried out by the Clinic Veterinarian.