Her-2/neu overexpressing breast cancer is associated with reduced overall survival, sex steroid receptor negativity and increased resistance to antihormonal therapy, and thus represents a subgroup with poor prognosis. The anti Her-2/neu receptor antibody trastuzumab (Herceptin), however, specifically targets this protein and provides a valuable addition to classical systemic therapies. Unfortunately, not all tumors that express Her-2/neu protein are also adequate candidates for trastuzumab therapy. Therefore, most clinicians now consider Her-2/neu oncoprotein overexpression and/or her-2/neu gene amplification a prerequisite for trastuzumab-based antineoplastic therapy. Nevertheless, due to the relatively low response rates that are observed even in this preselected patient cohort, better response predictors are clearly needed. Here, we review established parameters such as Her-2/neu immunohistochemistry (IHC) and fluorescence in situ hybridization (FISH) and their ability to predict the clinical course of trastuzumab-treated breast cancer. We also evaluate promising parameters such as serum levels of the Her-2/neu extracellular domain (ECD) and the activation status of Her-2/neu oncoprotein (pHer-2/neu), and their use in the clinical setting. Finally, novel tumor-specific such as tumor M2-PK, IGF-IR and p53 are discussed and their potential to predict the efficacy of trastuzumab is assessed.