Taking a series of repeat biopsies or fine needle aspirates of a tumor during the course of therapy can provide information about treatment-induced changes in tumor biomarkers and help monitor patient response to adjuvant therapy. It is hoped that analysis of biomarkers in serial biopsies will also further our understanding of the molecular mechanisms that determine a tumor’s response or resistance to therapy, may facilitate investigation of molecular biology of tumor response, and may provide useful information informing the development of new drugs for breast cancer therapy. In this chapter, practical, clinical considerations in the taking of repeat biopsies are considered and protocols for the taking of fine needle aspirates and core-cut/trucut biopsies are detailed. Their assessment for biomarkers indicative of cellular proliferation, apoptosis, and endocrine response/resistance such as estrogen and progesterone receptor status, HER2 and epidermal growth factor receptor are considered.