The classification of cellular fibroepithelial lesions of the breast can be diagnostically challenging.  In this lecture, we will review histologic features that enable the distinction of cellular fibroadenoma from phyllodes tumor and discuss the criteria for phyllodes tumor grading and current grading controversies. Potential diagnostic pitfalls, such as those related to proliferative stromal changes in adolescent patients, massive stromal overgrowth, architectural mimicry of neoplastic fronds, or aberrant immunohistochemical reactivity for epithelial markers, will also be presented.