Trastuzumab, a humanized monoclonal antibody against the HER-2 receptor, was the first targeted therapy for HER-2 positive breast cancer. The treatment of HER-2 positive breast cancer in monotherapy, but mainly in combination with cytostatics trastuzumab, showed significant efficacy and increased the time to progression/relapse and overall survival. On the other hand, despite it being administered to only a selected patient population, the response rate varies in range and duration due to the primary or acquired tumor resistance to trastuzumab. Several mechanisms contributing to the drug resistance are presumed and overcoming strategies are in development. In connection with the drug resistance there are the mechanisms of trastuzumab's action on tumor cells and the search for feasible biomarkers in order to predict the response of trastuzumab-based targeted therapies. Furthermore, we present here the actual possibilities to overcome primary or acquired trastuzumab resistance.