Currently, trastuzumab is one of the key drugs in the treatment strategy for HER2-positive breast cancer. Although metastatic breast cancer is unlikely to be cured, some HER2-positive patients have survived for a long time with complete response to trastuzumab therapy. In HER2-positive metastatic breast cancer, single agent trastuzumab is effective and less toxic. Combination of trastuzumab with cytotoxic drugs is also effective. Therefore, major guidelines recommend using trastuzumab as a key drug in the management of HER2-positive metastatic breast cancer. However, many clinical questions still need to be solved. In this article, recent evidence was reviewed to find some answers about these issues.