Approximately one in eight U.S. women will develop breast cancer during her lifetime. Because the earliest stages of breast cancer are asymptomatic, the process of breast cancer diagnosis is often initiated by detecting an abnormality through self-examination, physical examination by a clinician, or screening mammography. If the initial assessment suggests that the abnormality could be breast cancer, the woman is likely to be referred for a biopsy-a sampling of cells or tissue from the suspicious lesion. Three techniques for obtaining samples from suspicious breast lesions are available: fine-needle aspiration, biopsy with a hollow core needle, or open surgical retrieval of tissue. Fine-needle aspiration samples cells and does not assess tissue architecture, is generally considered less sensitive than core needle and open biopsy methods, and is used less frequently. Core-needle biopsy, which retrieves a sample of tissue, and open surgical procedures are the most frequently used biopsy methods. Lesion samples obtained by core needle or surgical biopsy are evaluated by pathologists and classified into histological categories with the primary goal of determining whether the lesion is benign or malignant. Because core needle biopsy samples only part of the breast abnormality, a risk exists that a lesion will be classified as benign, high risk, or noninvasive when invasive cancer is in fact present in unsampled areas. Open surgical biopsy samples most or all of the lesion, and is therefore considered to have a smaller risk of misdiagnosis. However, open procedures may carry a higher risk of complications, such as bleeding or infection, compared to core needle biopsy procedures.5 Therefore, if core needle biopsy is also highly accurate, women and their clinicians may prefer some type of core needle biopsy to open surgical biopsy. In 2009, the ECRI Evidence-based Practice Center conducted a comparative effectiveness review for core needle versus open surgical biopsy commissioned by the Agency for Health Care Research and Quality. That evidence report assessed the diagnostic test performance and adverse events of core needle biopsy techniques compared to open surgical biopsy and evaluated differences between open biopsy and core needle biopsy with regards to patient preferences, costs, availability, and other factors. The authors concluded that core needle biopsies were almost as accurate as open surgical biopsies, had a lower risk of severe complications, and were associated with fewer subsequent surgical procedures. The publication of additional studies and changes in practice raised the concern that the conclusions of the original report may be out of date. New studies may also provide additional information allowing the exploration of heterogeneity for test performance and safety outcomes. Therefore, an updated review of the published literature was considered necessary to synthesize all evidence on currently available methods for core needle and open surgical breast lesion biopsy. We specified the following three Key Questions to guide the conduct of the update: Key Question 1: In women with a palpable or nonpalpable breast abnormality, what is the test performance of different types of core needle breast biopsy compared with open biopsy for diagnosis? Key Question 2: In women with a palpable or nonpalpable breast abnormality, what are the adverse events (harms) associated with different types of core needle breast biopsy compared with open biopsy for diagnosis? Key Question 3: How do open biopsy and various core needle techniques differ in terms of patient preference, availability, costs, availability of qualified pathologist interpretations, and other factors that may influence choice of a particular technique?