Immunohistochemical techniques detect antigens in tissue sections by means of immunological and chemical reactions. This technique is highly sensitive and specific and can detect a wide variety of antigens in multiple animal species. This chapter reviews common immunohistochemical methods used in the characterization of normal and pathologic tissue and the reagents used. Pretreatments such as blocking steps for endogenous activities and antigen retrieval are included. Standard procedures on formalin-fixed, paraffin-embedded tissues as well as method standardization for new antibodies and troubleshooting are emphasized.