Amplification and overexpression of the Her2/neu (c-erbB-2) growth factor receptor occurs in approximately 25% of early stage breast cancers. HER2/neu has been established as an important prognostic factor in early stage breast cancer in large patient populations and has been associated with shorter overall and disease free survival. New data are emerging to suggest that HER2/neu may be useful not only as a prognostic factor but also as a predictive marker for response to chemotherapeutics, anti-estrogens, and therapeutic regimens using anti-HER2/neu monoclonal antibodies. However, little is known of how other erbB receptors tyrosine kinases affect breast cancer biological behavior and response to therapy. In this review, we highlight recent data on Her2/neu as a prognostic and predictive marker of response to therapy, as well as how the other receptors of the erbB family affect the biological behavior of breast cancer.