The microanatomical and histological appearance of the human breast has been studied during puberty. The macroscopic architecture of the mammary epithelial tree was identified and correlated with the histological appearance of material excised from defined regions of the breast preparations. Between ages 13 yrs and 15 yrs the human breast shows evidence of ductal elongation and branching, with lobules formed by lateral and dichotomous branching. The majority of ducts are lined by a two-layered epithelium consisting of recognisable myoepithelial and luminal cells. Less-well-defined multilayered regions were also observed in some areas, apparently at the site of lateral branching or early lobular development.