The HER2 protein is encoded by the HER2/neu gene and it is homologous to the epidermal growth factor receptor. Overexpression of HER2, usually in association with gene amplification, occurs in approximately 25-30% of breast cancers. There are currently several different methods available to evaluate HER2 status, e.g. immunohistochemical (IHC), and fluorescence in situ hybridization (FISH) assays. The HER2 protein is a viable therapeutic target. The humanized anti-HER2 monoclonal antibody trastuzumab (Herceptin) has demonstrated activity in clinical trials in women with metastatic breast cancer overexpressing HER2. The mechanisms of the action of this antibody involve disruption of DNA repair and induction of antibody-dependent cellular cytotoxicity. Response rates to the antibody given as a single agents in the treatment of HER2 overexpressing metastatic breast cancer have ranged from 12 to 27%. Patients who received trastuzumab in combination with chemotherapy had a significantly longer time to progression, higher overall survival compared with patients who had received chemotherapy alone. In the treatment of women with HER2 overexpressing tumors an overall response rate of 57% for combination trastuzumab plus paclitaxel compared with 25% for paclitaxel alone was found. Trastuzumab has an important role in the treatment of HER2 overexpressing metastatic breast cancer. Its place in adjuvant treatment has not been proved up to now. The optimal use of trastuzumab in the treatment of HER2 positive advanced breast cancer is under active investigation. Due to the high rate of clinical activity and low incidence of severe toxicity trastuzumab is a very promising drug in the treatment of breast cancer. The author's purpose was to summarize the results of the trials using trastuzumab treatment, and discuss the methods used to determine the HER2 status.