Normal tissue section is colourless as the fixed protein has the same refractive index as that of glass. Dyes are used to impart colour to the different components of the tissue for the interpretation. A chromogenic dye absorbs the light of particular wavelength of the white light representing a specific colour and emits the light containing the rest of the colour. Therefore we see a coloured light from the dye. The dye can be classified on the basis of electrical charge or chemical structure. The staining is the combination of a coloured dye with the tissue that retains the dye after washing. It is primarily a chemical reaction between the dye and the tissue, and the common chemical reactions are electrostatic bond, van der Waals attractions, hydrogen bond, covalent bond, hydrophobic bond and dye aggregations. The chapter presents the classification of dyes, principle of staining, mechanisms of different dyes and factors that affect the staining. In addition, an overview of staining procedures has also been described here.