The prognosis of breast cancer has improved dramatically due to advances in drug therapy, and cure has been achieved in many cases of early-stage breast cancer. Drug therapy should always be administered with a balance of efficacy and toxicity in mind. Effective drugs and suitable patients should be selected from the viewpoint of drug sensitivity, and baseline risk (disease progression, patient's general condition, age, living environment, etc.) and toxicity (adverse events, cost) of patients should be considered. In the field of breast cancer, drug escalation has resulted in prolonged prognosis, while many de-escalation therapies with reduced toxicity have been developed. Prognosis prediction using multi-gene assays such as Oncotype DX and MammaPrint, and benefit prediction of chemotherapy are examples of such treatments, which are useful for selecting patients who do not need chemotherapy. In addition, trials of chemotherapy omission in elderly patients who are vulnerable to toxicity (RESPECT trial) and trials of treatment selection with reduced toxicity (HERB-TEA trial) are being attempted. Reducing the duration and cost of treatment is also an important issue. In the case of breast cancer, it is also necessary to consider fertility preservation in young women. In order to achieve these goals, it is also necessary to develop individualized treatment using further technologies such as ctDNA and gene panel testing.