Immunocytochemistry as a technique has had a tremendous impact on the development of neurobiology in the last decade. It is not a new technique. The principle that antibodies can be used as histochemical reagents was recognized early by Marrack (1934), showing its applicability if suitable markers were linked to antibody molecules. Of the various markers that have subsequently been examined, only the fluorescent dyes have provided the sensitivity necessary. The first example was fluorescein isothiocyanate-labeled antibodies that were employed to localize pneumococcal antigens in infected tissues (Coons et al., 1942). Another example was the localization of adrenocorticotrophic hormone using fluorescein-labeled antibodies raised against a partially purified antigen (Marshall, 1951). Localization of endogenous antigens in tissues was thus accomplished by the “indirect” immunocytochemical technique (Coons et al., 1955; Mellors et al., 1955). Another procedure involved the use of horseradish peroxidase, which resulted in the elegant peroxidase-antiperoxidase (PAP) technique (Sternberger et al., 1970).