This lecture is designed to introduce diseases of the breast and to familiarize second year medical students with common benign and malignant lesions they are likely to encounter in clinical practice. Within this lecture, particular attention is paid to clinical pathologic correlations. An emphasis is placed on the clinical differential diagnosis of palpable lesions with correlations between pathologic findings and clinical findings. An introduction to the nomenclature of epithelial tumors of the breast is provided with an emphasis on clinical and prognostic features that predict patient outcomes. From the Series:Pathology of the Breast