A useful prognostic factor in breast cancer has key roles, including identification of a group of patients whose prognosis is so good they do not require further treatment, such as adjuvant systemic therapy, after local surgery, and secondly a group with a poor prognosis for whom additional treatment would be appropriate. To be of clinical use, prognostic factors must show a wide separation in the outcome of the groups identified and select adequate numbers in each group. No single prognostic factor in invasive carcinoma of the breast satisfies all these criteria. However, the Nottingham prognostic index (NPI), which combines nodal status, tumour size and histological grade, does satisfy these criteria. The NPI has been validated by further studies in Nottingham and by studies in several other countries. Predictive factors, such as oestrogen receptor and HER-2 status, predict whether a tumour is likely to respond to a treatment, and are complimentary to prognostic factors. The NPI can be used in combination with predictive factors to select patients for systemic adjuvant treatments. There is the potential to improve the NPI by inclusion of other factors, such as vascular invasion, but any such alterations would require further validation.