Trastuzumab (Herceptin) has many benefits for metastatic breast cancer patients with HER2 overexpression/amplification. Trastuzumab alone or trastuzumab in combination with chemotherapy regimens are standard treatment worldwide as first line therapy for metastatic breast cancer patients with HER2 overexpression/amplification. Furthermore, an international collaboration for adjuvant trastuzumab trials showed last year that trastuzumab treatment improves disease-free and overall survival after or in combination with adjuvant chemotherapy. However, there are many uncertain issues concerning trastuzumab adjuvant and metastatic treatment, such as treatment beyond disease progression (PD), combination with hormone therapy, duration of adjuvant treatment, and cardiac safety of long term treatment.