The WHO Classification of Tumors of the Breast, 4th edition, is an update to the 3rd edition that was published in 2003, and covers all neoplastic and preneoplastic lesions of the breast. Changes to the 4th edition include new aspects and changes to the terminology that reflect our present-day knowledge of these lesions. Definitions for histopathological diagnosis are complemented by a description of clinical features, epidemiology, macroscopy, genetics, and prognostic and predictive features. In this review of the WHO classification, we have focused on invasive carcinomas, precursor lesions, and some benign epithelial proliferations.