If the benefit of adjuvant chemotherapy may be determined at the level of a population, to determine the real chemosensitivity of a tumor at the individual level is impossible. The concept of neoadjuvant chemotherapy in patients with localized breast cancer is interesting because it helps to know the chemosensitivity of a tumor "in vivo". It is possible to use a single criterion to predict the effectiveness of targeted therapies. The chemotherapy is not a targeted therapy, and to determine a biological predictive marker of the response has been impossible so far. The development of mathematical models and use of molecular biology may help to predict chemosensitivity. Initial results are promising. The validation of published works is necessary, but applications are numerous.