The purpose of this course is to discuss diagnostic problems in breast pathology encountered by pathologists and pathologists-in-training in their daily practice, and to provide practical guidelines and criteria that are useful for their resolution. The uses and limitations of immunohistochemistry in helping to resolve these diagnostic problems will be emphasized, where appropriate. We will use a case-oriented/slide seminar-type approach to address a variety of common diagnostic problems which, based on our consultation practice experience, seem to be a continual source of difficulty for pathologists, including: the distinction of benign and in situ lesions from invasive lesions (including a discussion of the uses and limitations of myoepithelial cell markers to resolve these problems); carcinomas in situ with histologically ambiguous features (including a discussion of the uses and limitations of immunostaining for E-cadherin and high molecular weight cytokeratin); evaluation of metastatic lesions for possible breast origin (the role of immunostaining for ER, GCDFP, CK7/20, TTF-1, etc, to aid in the diagnosis will be discussed here); papillary lesions (distinguishing benign papilloma and papilloma variants from atypical papilloma from papillary DCIS); fibroepithelial lesions (fibroadenoma vs phyllodes tumor; benign vs borderline vs malignant phyllodes tumor); columnar cell lesions and flat epithelial atypai; spindle cell lesions (and the role of immunohistochemistry in their evaluation); sentinel lymph node biopsy and axillary micrometastases; and vascular and pseudovascular lesions. At the end of this course, registrants should be able to more effectively deal with and resolve these common problems in breast pathology. (Last Scheduled Presentation)