Although immunohistochemical detection of the Ki67 antigen has been used for many years to assess cancer proliferation, this marker is still not recommended for routine use in clinical management of breast cancer. The major reason for this situation is a lack of a standardised procedure for Ki67 assessment as well as persistence of several issues of debate with regards to the Ki67 assay interpretation and the marker's clinical utility. Nowadays Ki67 assessment is principally used for estimation of prognosis and guiding the decision on adjuvant treatment choice, as well as for prediction of response to neoadjuvant treatment in ER+/HER2- breast cancer. In ER-/HER2+ and ER-/HER2- tumours, high post-neoadjuvant Ki67 index is associated with unfavourable prognosis. We review here the elements impacting analytical validity of the Ki67 immunohistochemical assay, the evidence of its clinical utility and the current recommendations for its use in breast cancer management.