Histological dyes are used whenever defined intra- or extracellular elements have to be displayed. According to their sources, coloring agents are discerned as natural or as synthetic dyes. So-called general stains will dye the tissue uniformly (indifferently) while selective stains have affinity for special cell or tissue components. Furthermore, dyes can be roughly divided into acidic dyes and basic dyes. Basic dyes stain acidic components, and acidic dyes stain mainly basic components. Some systematic approach to dye nomenclature provided a means of identification by the Colour Index Number for each individual dye which was assigned on the basis of chemical structure. A uniform theory of histological dye staining does not exist. This is because the mechanisms of dye binding to the different tissue components are quite heterogenous. Hence, histological dye methods cannot be interpreted as an isolated dye reaction, but must be seen in connection with the chemical and morphological behaviour of the stained structures. The process of staining can rely on both chemical or physical grounds.