The success of needle core biopsy procedures and the validity of pathologic diagnoses made on material from the procedures are key determinants in planning the optimal management of a wide variety of breast diseases. The most common diagnostic problems encountered in these biopsy specimens include lobular and ductal proliferations, sclerosing and papillary lesions, cellular fibroepithelial tumors, and minimally invasive or microinvasive carcinoma. This review provides practical guidance to help narrow the differential diagnosis of lesions on needle core biopsies and offers guidelines for the pathologic reporting of these specimens.