The HER-2/neu oncogene is located on chromosome 17q and encodes a transmembrane glycoprotein with intracellular tyrosine kinase activity. Several studies have shown an association of HER-2 gene amplification or protein overexpression with prognosis and predictor of therapeutic response. Most important, the presence of amplification or overexpression is the basis of eligibility for trastuzumab therapy. However, there are several methods of determining HER-2 status, each measuring a different aspect such as DNA content, gene copy number, protein expression, expression of RNA, and circulating HER-2 extracellular domain protein. There is no consensus with regard to the optimal test for HER-2 assessment. This review examines the various methods used in an attempt to define the most accurate and reliable test. The most widely used assays are immunohistochemical analysis and fluorescence in situ hybridization (FISH), which measure protein expression and gene amplification, respectively. Based on current data, FISH is the most accurate and reproducible test with a better correlation with prognosis and response to therapy.