HER2 gene is overexpressed or amplified in approximately 30% of breast cancer. Breast cancer patients with HER2-overexpression or amplification have shortened disease free and overall survival. The HER2 protein is a useful target for antibody therapy of cancers overexpressing the HER2 gene. Since the recombinant humanized anti-HER2 monoclonal antibody, trastuzumab (Herceptin) was introduced into clinical practice, trastuzumab has become one of the key drugs in the treatment for HER2 positive metastatic breast cancer(MBC). However, much controversy exists on the optimal use of trastuzumab. In this paper, positioning of trastuzumab in the treatment algorithm for HER2 positive MBC as well as early breast cancer will be reviewed and discussed on the basis of our experience.