Breast cancer is the most common malignancy in women in the United States in the year 2000. The proto-oncogene Her-2/ neu (c-erb-B2) has become an increasingly important prognostic and predictive factor in breast cancer. Overexpression/amplification of the Her-2/ neu has been associated with a worse outcome in patients with breast cancer. Herceptin, a “humanized” murine monoclonal antibody directed against the extracellular domain of the Her-2/ neu protein, is being used to treat breast cancer that overexpresses Her-2/ neu. The status of Her-2/ neu in the tumor has become a critical factor in the management strategy of a breast cancer patient. The objective of this article is to provide a comprehensive review of all aspects of Her-2/ neu in breast cancer, including biology, prognostic and predictive value, targeted Herceptin therapy, and the laboratory testing of Her-2/ neu.