This compilation of manuscripts encompass a breadth of information from different species including rodents, dogs, cows, goats, and sheep. Normal developmental processes are covered and anatomical features are discussed, as well as the impact and significance of environmental chemicals and epigenetics on the mammary gland. In terms of disease states, cancer and mastitis are the most impactful in the fields of human medicine and livestock management, respectively, in the context of mammary gland biology.