Targeted drugs against breast cancer are being developed centering around ones targeting HER2. In addition to trastuzumab and lapatinib that were already approved in Japan, promising drugs are being developed one after another. Basic mechanisms of their action and resistance emergence are being clarified step by step. In consideration of extraordinarily high medical expenditures of targeted drugs compared with conventional cytotoxic anticancer agents, it is critical to choose a targeted drug from a standpoint that it has definitely different features besides high efficacy and it has predictive factors which enable accurate anticipation of effects or adverse events. It will be confusing for the time being how each targeted drug is used properly in terms of the prescription order or combinations with chemotherapy. Over the middle- or long-term, clinical trials comparing these drugs will be conducted until clinically effective and easy-to-use targeted drugs in a true sense survive. Clinical handling of targeted drugs is not easy if conducted in the conventional way. Whereas conventional cytotoxic anticancer agents have major adverse events like nausea/vomiting or myelosuppression in common, such a variety of side effects occur in the use of targeted drugs as vary greatly according to different mechanisms of action of individual drugs. Oncologists will be required to master appropriate initial therapy to treat them; on the other hand, oncologists need to cooperate with specialists in other fields more than they used to.