Breast cancer is the predominant malignancy where oncologists use predictive markers clinically to select treatment options, with steroid receptors having been used for many years. Immunohistochemistry has taken over as the major assay method used for assessing markers. Despite its extensive use there are still issues around tissue fixation, methodology, interpretation and quantification. Although many markers have been evaluated, the oestrogen receptor remains the most reliable and best example of a predictor of treatment response. It is of major importance clinically that those undertaking interpretation of predictive markers understand the technical pitfalls and are aware of how expression of a particular marker relates to breast cancer pathology. A false negative or a false positive result will impact on patient management.