These guidelines update the previous UK HER2 testing guidelines and have been formulated to give advice on methodology, interpretation and quality assurance to ensure that HER2 testing results are accurate, reliable and timely with the expansion of testing to all patients with breast cancer at the time of primary diagnosis. The recommendations for testing are the use of immunohistochemistry but with analysis of equivocal cases by in situ hybridisation to clarify their HER2 status or the use of frontline fluorescence in situ hybridisation (FISH) testing for those laboratories wishing to do so; the inclusion of a chromosome 17 probe is strongly recommended. Laboratories using chromogenic or silver in situ hybridisation should perform an initial validation against FISH. For immunohistochemistry and in situ hybridisation there must be participation in the appropriate National External Quality Assurance scheme.