Immunohistochemistry is an analytical technique that applies an antibody reagent to detect and visualize an antigen in cytological and surgical pathology microscopy specimens in the context of histomorphology and cytomorphology. The clinical-pathological interpretation of the presence and patterns of the antibody-antigen reactions is performed in a manner similar to other molecular pathology assays. Immunohistochemistry is used in diagnostic pathology for diagnosis, determination of prognosis, and predictive assays for response to therapy. Accurate and reproducible results require quality assurance of the total test system including the design control of the reagents and the preexamination (preanalytical), examination (analytical), and postexamination (postanalytical) interpretation steps (processes) of the assay to ensure its clinical applicability. This guideline focuses on validation of immunohistochemistry assays on formalin-fixed, paraffin-embedded pathology material. The audience for this guideline includes the assay developer, the reagent supplier, laboratory histotechnologist who performs the assay, and the laboratory director/pathologist who implements and interprets the assay.