Since the field of immunocytochemistry was pioneered almost half a century ago, over 30 different immunostaining methods have been developed. Most laboratory investigators, however, are aware of only a few of the more popular techniques. This article, therefore, reviews and places in historical perspective many of the less common methodologies. A discussion of the progression of immunostaining methodology from purely immunologic techniques (employing only antigens, antibodies, and free enzymatic preparations) to procedures that use non-immunologic substances (such as avidin, biotin, and protein A) is presented. Basic principles of immunostaining, including specimen preparation, fixation, control procedures, interpretation, and problems unique to cytopathology, are also discussed. Schematic diagrams that illustrate the nature and arrangement of the immunochemical reagents employed in most of these techniques are provided in order to demonstrate how various materials can be combined to create a desired staining effect.