The mature human female breast contains thousands of hormone-sensitive, potentially milk-producing microorgans, called lobules. In the nonpregnant state they vary in size from 1 to 8 mm but most are 1 to 2 mm in diameter. Each lobule is drained by a terminal duct attached to the main duct system. It is called the terminal ductal-lobular unit, which normally regresses at menopause. Most breast diseases except papillomas in major ducts arise in terminal ductal-lobular units. Disease processes such as hyperplasias and neoplasia alter the terminal ductal-lobular units to such a degree that conventional light microscopic appearances make the structures appear as though they were ducts. However, the basic microarchitecture in three dimensions is preserved, proving the origin of cancer is the terminal ductal-lobular unit.