The NCCN HER2 Testing in Breast Cancer Task Force was convened to critically evaluate the ability of the level of HER2 expression or gene amplification in breast cancer tumors to serve as a prognostic and a predictive factor in the metastatic and adjuvant settings, to assess the reliability of the methods of measuring HER2 expression or gene amplification in the laboratory, and to make recommendations regarding the interpretation of test results. The Task Force is a multidisciplinary panel of 24 experts in breast cancer representing the disciplines of medical oncology, pathology, radiation oncology, surgical oncology, epidemiology, and patient advocacy. Invited members included members of the NCCN Breast Cancer Panel and other needed experts selected solely by the NCCN. During a 2-day meeting, individual task force members provided didactic presentations critically evaluating important aspects of HER2 biology and epidemiology: HER2 as a prognostic and predictive factor; results from clinical trials in which trastuzumab was used as a targeted therapy against HER2 in the adjuvant and metastatic settings; the available testing methodologies for HER2, including sensitivity, specificity, and ability to provide prognostic and predictive information; and the principles on which HER2 testing should be based. Each task force member was charged with identifying evidence relevant to their specific expertise and presentation. Following the presentations, an evidence-based consensus approach was used to formulate recommendations relating to the pathologic and clinical application of the evidence to breast cancer patient evaluation and care. In areas of controversy, this process extended beyond the meeting to achieve consensus. The Task Force concluded that accurate assignment of the HER2 status of invasive breast cancer is essential to clinical decision making in the treatment of breast cancer in both adjuvant and metastatic settings. Formal validation and concordance testing should be performed and reported by laboratories performing HER2 testing for clinical purposes. If appropriate quality control/assurance procedures are in place, either immunohistochemistry (IHC) or fluorescence in situ hybridization (FISH) methods may be used. A tumor with an IHC score of 0 or 1+, an average HER2 gene/chromosome 17 ratio of less than 1.8, or an average number of HER2 gene copies/cell of 4 or less as determined by FISH is considered to be HER2 negative. A tumor with an IHC score of 3+, an average HER2 gene/chromosome 17 ratio of greater than 2.2 by FISH, or an average number of HER2 gene copies/cell of 6 or greater is considered HER2 positive. A tumor with an IHC score of 2+ should be further tested using FISH, with HER2 status determined by the FISH result. Tumor samples with an average HER2 gene/chromosome ratio of 1.8 to 2.2 or average number of HER2 gene copies/cell in the range of greater than 4 to less than 6 are considered to be borderline, and strategies to assign the HER2 status of such samples are proposed.