The ERBB2 proto-oncogene, commonly referred to as the human epidermal growth factor receptor-2 (HER2) gene, encodes a 185 kd receptor tyrosine kinase. Overexpression of the protein leads to constitutive activity of the HER2 receptor and breast tumor development through enhanced cell proliferation, survival, motility and adhesion. Overabundance of the HER2 receptor, typically caused by amplification of the HER2 gene, is present in approximately 10-30% of invasive breast cancers, and is associated with an aggressive disease course and decreased disease-free and overall survival in node-positive patients. Tratuzumab, a humanized murine monoclonal antibody, offers a targeted treatment modality for tumors that over express the HER2 protein. Tratuzumab, shown to be effective and initially approved for treatment of metastatic breast cancer, has recently been shown to be very effective in the adjuvant setting. Thus, to offer prognostic information and to direct appropriate treatment it is important to provide accurate laboratory assessment of the status of HER2. This article provides an overview of the methods currently used to assess HER2.