Immunotherapy is a promising new approach in breast cancer treatment, complementing surgery, chemotherapy, radiation and antihormonal therapy. Primarily, two treatment options are available which are supported by a rapidly growing body of clinical evidence. These are active specific immunotherapy with Theratope and passive immunotherapy targeting the HER-2 receptor with trastuzumab (Herceptin). Trastuzumab has a proven efficacy as monotherapy as well as in combination with chemotherapeutic agents in HER-2-overexpressing metastatic breast cancer. Trastuzumab is generally well tolerated although cardiotoxicity has been observed, especially when in combination with doxorubicin (Adriamycin), where this can be a serious concern. Therefore, less cardiotoxic combinations with docetaxel (Taxotere), vinorelbine (Navelbine) and epirubicin (Pharmorubicin) or cyclophosphamide (Endoxana) have been tested. The combination of trastuzumab with paclitaxel (Taxol) is well approved. The use of trastuzumab in adjuvant and preoperative therapy is currently being examined in controlled trials. After a brief outline of immunotherapy with Theratope and trastuzumab, this article reviews recent and ongoing clinical studies conducted with trastuzumab.