Amplification of the HER-2/neu (c-erbB-2) gene resulting in overexpression of the p185HER-2 growth factor receptor occurs in ~25% of early stage breast cancers. HER- 2/neu has been established as an important independent prognostic factor in early stage breast cancer in large cohorts of patients and in cohorts with very long (30 year) follow-up duration. New data are emerging to suggest that HER-2/neu may be useful not only as a prognostic factor but also as a predictive marker for projecting response to chemotherapeutics, antiestrogens, and therapeutic anti-HER- 2/neu monoclonal antibodies. In this review we highlight recent data on HER-2/neu as a predictive marker of response to breast cancer therapy and discuss the clinical implications of this information. The difficulty in comparing results from different data sets due to the wide variety of reagents and technologies used to detect HER-2/neu amplification/overexpression in clinical specimens is also discussed. Finally, we report results from experimental models of HER-2/neu overexpression which have been used in an effort to understand the relationship between HER- 2/neu and response to chemotherapeutics and antiestrogens in breast cancer.