In the age of personalized medicine, and in addition to typing and grading, breast cancer pathologists are now also involved in determining biomarkers such as steroid hormone receptors and Her-2, which are of the utmost importance in adjuvant therapy. In order to assure quality of these biomarker assays, external proficiency testing has been implemented in Germany. Since 2002 trials have been conducted annually, with up to 180 participating laboratories. More than 85% of all participants achieved good results in clearly negative and positive cases seen in daily practice. If at all, discordant results were observed in the rarer low steroid-hormone receptor expressing tumors and Her-2 borderline cases (2+). Regular participation in interlaboratory testing leads to significantly improved immunohistochemical results, particularly in these problematic cases. Tissue microarrays (TMA) with 20-24 different breast cancer samples including cell lines meant that a huge number of pathologists were challenged with identical samples, providing the prerequisite for comparability. Participation is recommended for pathology departments involved in the service for breast units. The organizational frame work of the trials is described here. The confidence of cooperating disciplines in breast cancer biomarkers assessed by pathologists will be fostered by external proficiency testing as presented here.