From the beginning of selective cell labeling by use of fluorochrome conjugated antibodies until today with all the further developments in immunohistology, histologists are called for a number of control reactions to prove the specificity of their staining reactions. This very special technology with its great opportunities is technically complex. With all the newer developments and innovations in the last ten years, immunohistology has become more and more common in histopathology. In practice, however, immunohistological reagents are often applied just as “special” stains (much comparable to classical histological stains). Because histologists are more interested in morphology and not so in adequate care of the reagents by avoiding the rigors of quality assurance, extensive quality control procedures, reagent validation and documentation as is conventional in the clinical laboratory are not applied. Consequently, a high level of disagreement of histopathologic diagnoses may be found in observer studies.