Adjuvant chemotherapy is of significant survival benefit in women aged < 70 years with moderate to high risk breast cancer. The benefits seem to be greater in younger women. This is mainly because of biological factors, including a higher incidence of grade III and hormone receptor negative cancers. Ovarian failure induced by chemotherapy is also likely to be a factor in women with oestrogen receptor positive tumours. Chemotherapy benefit is also seen in high risk women aged > 50 years, but for many of these women endocrine therapy alone may be just as effective, except when the tumour is grade III, tests positive for human epidermal growth factor receptor 2 (HER2), or there is multiple node involvement. Risk benefit considerations are always important here because of toxicity, and consensus criteria have been defined to aid in selecting patients for treatment.