Determining ErbB2/Her-2/neu status has become an essential part of breast cancer diagnosis and a prerequisite before considering a patient's eligibility for treatment with trastuzumab. Currently the most common techniques to assess ErbB2 status in routine practice are the identification of receptor overexpression by means of immunohistochemistry (IHC) and the analysis of gene amplification by means of dual color fluorescence in situ hybridisation (FISH). According to recent recommendations ("ASCO/CAP Guidelines" and German S3 guidelines for breast cancer) the choice of primary test procedure--IHC or ISH - is left to the individual institution. Both techniques are of equal predictive value provided that strict quality precautions have been taken: internal test validation by comparing IHC and (F)ISH, carrying out controls, and annual participation in round-robin tests. Equivocal IHC (score 2+) has to be checked by ISH for amplification. Borderline ISH (ratio 1.8-2.2 or gene copy number 4.0-6.0) should be retested by counting additional cells or performing IHC. In approximately 5% of cases these criteria give conflicting results and the gene copy number alone generates over 90% of the equivocal ISH cases, mostly due to chromosome 17 polysomy. These cases need to be tested by IHC since over-expression is very exceptional and only these tumors have the potential to be trastuzumab responders.