Spontaneous mammary tumours are most frequently seen (apart from rodents) in women, female dogs and cats. The mammary gland is the most commonly affected organ for tumours in women and in female dogs. The mammary gland has a similar histology in the different species whereas the number of glands differs as well as the number of interlobular ducts that reach the nipple/teat. The parenchymatous tissue is composed of alveoli that turn into interlobular ducts. The whole ductal tree is outlined by a two-layered epithelium with the luminal epithelial cells adjacent to the lumen and the more sparse myoepithelial cells peripherally located to these. Different proteins such as growth factors regulate the mammary gland, as they do for all tissues in the body. In addition, sex hormones regulate the biology of the mammary gland. Oestrogen has the most pronounced effect on duct growth whereas progesterone promotes growth of the alveoli.