Adjuvant chemotherapy, initiated following surgery, aims at controlling "microscopic" metastatic disease before its progression to a "clinical" recurrence. Its efficiency in localized breast cancer was officially demonstrated in 1992 in the Early Breast Cancer Trialists' Collaborative Group meta-analysis. This analysis helped clarify the benefits of therapeutic interventions according to progression stage, age and tumour biological features, especially in the presence of hormone receptors. Later on, new trials further defined the benefits of modern therapies, and especially trastuzumab in patients overexpressing HER2. The assessment of the theoretical benefits of adjuvant therapies takes into account the risk of individual recurrence and the potential decrease of the latter following a given therapeutic intervention--this decrease depending on tumor biological features. Deciding whether an adjuvant therapy should be initiated is difficult since benefits (observed in the long term) are often moderate while toxicities (immediately observed) are significant. The development of national and regional therapeutic guidelines markedly diminished the arbitrary aspect of the decision. Developing tools which could help implying the patient in this therapeutic decision is a priority.