In this review of immunohistochemical methods, an emphasis is placed on the historic development of enzyme-linked procedures that are applicable to antigen localization in routinely processed tissues. As the technology of immunohistochemistry has improved, a bewildering montage of antibodies, bridges, labels, and chromogens has evolved. Each has its apparent advantages, yet each also has certain limitations. Admittedly, a comprehensive review of these different methods is beyond the scope of this forum, and several immunohistochemical techniques in current use will not be discussed in detail. Still, the principles of horseradish peroxidase-linked procedures, as described in this review, form the foundation of contemporary diagnostic immunohistology. They provide not only a powerful method of antigen detection, but also a model with which to compare recent and future innovations.