Current treatment of breast cancer incorporates clinical, pathological and molecular data. Oestrogen receptor (ER), progesterone receptor (PR) and human epidermal growth factor receptor 2 (HER2) define prognosis and identify tumours for targeted therapy, and remain the sole established single-molecule biomarkers defining the minimum breast cancer pathology data set. Ki67 remains one of the most promising yet controversial biomarkers in breast cancer, implemented routinely in some, but not all, pathology departments. Beyond the single-molecule biomarkers, a host of multigene expression tests have been developed to interrogate the driver pathways and biology of individual breast cancers to predict clinical outcome more accurately. A minority of these assays have entered into clinical practice. This review focuses on the established biomarkers of ER, PR and HER2, the controversial but clinically implemented biomarker Ki67 and the currently marketed gene expression signatures.