Immunohistology and a wide range of other immunological techniques depend on the use of labeled antibodies. A number of various labels such as radiolabels, fluorophores or enzymes exist and can be used as reporters in distinct assay formats. Despite the usefulness of fluorescent labels one would like to possess alternative methods. The development of enzyme markers for immuno-staining, and here especially the use of horseradish peroxidase (HRP), was a progress in immunohistology which has greatly facilitated all immunohistological investigations. In principle, all enzymes can be used in light or electron microscopy which produce, upon action on their substrates, colored and insoluble or electron dense products. Other marker molecules such as colloidal gold particles also proved useful. Those markers are especially useful in electron microscopy (EM postembedding immuno-stainings) and in EM-cryotechniques.