Evaluation of tissue and cellular samples for diagnosis, risk assessment, and prognosis in breast cancer is the subject of this review. We emphasize indicators of elevated risk for breast cancer and carcinomasin situand indicators of good prognosis in invasive breast cancer. The importance of ductal carcinomain situto considerations in breast conservation and prevention is highlighted. Special types of breast cancer, immunohistochemistry, histologic grading, and the relevance of core biopsy to diagnostic certainty are considered. We also add a brief note about the escalating role of nodal micrometastases and sentinel node biopsy in the definition of minimal regional disease.