Azo (A) -hydrazone (H) tautomerism is a feature of azo compounds in which the hydroxy group is conjugated to the azo group, being either o-or p-position. Whether the tautomerism is actually observed depends on number of structural and environmental factors such as aromaticity of the ring containing the OH group, substituents in the aromatic rings, temperature, solvents and irradiation with light. Depending on the particular dye structure, the pH (as in aminoazo dyes ) or addition of metal ions (as in macrocycle containing azo dyes ) can also affect the tautomeric state. Among them the resonance stabilization energy plays the decisive role going from A to H, this the reason why the azophenols exist as single A-tautomers as a rule. The tautomerism as a phenomenon is not only of crucial importance for the dyestuff manufacturers but also to other areas of chemistry. The azo and hydrazone tautomers not only have different colours, they also have different tinctorial strengths, and different properties, such as light fastness and chemical stability. Since usually the H form, due to the longer chromophore system and stronger charge transfer nature, is bathochromic and hyperchromic compared to the A tautomer, it has higher tinctorial strength, which is the reason why commercial hydroxyazo dyes have been chosen to exist predominantly, if not exclusively, in the hydrazone form. Compound 1 does not have substantial commercial value due to the sensitivity of the tautomeric equilibrium to various structural and environmental (especially the easy ionization of the unprotected OH group in solution) effects. The same reason makes it one of the most studied model tautomeric systems, where the effects of the substituents in the phenyl ring and the effect of the solvent are revealed. The intramolecular hydrogen-bonding in the ohydroxyazo dyes 2 and 3 decreases the effects of the environment and the fact that in both compounds the hydrazo form is, if not dominating, substantially presented leads to their widespread commercial use. Their existence in the tinctorially stronger hydrazone form, their pH and solvent insensitivity as a result of the intramolecular hydrogen bonding, make them ideal colorants. The derivatives of 2 have been used primarily as pigments, while sulphonated derivatives of 3 have found extensive use in most dyestuff classes as direct dyes for cotton, nylon and wool. The information about tautomerism in 1-3 have been presented is several reviews and various aspects of it have been discussed in more general books on dyes and solvent effects. According to various estimates the percentage of the tautomeric structures among the commercial azo dyes is substantial, which explains the continuous scientific and practical interest in this phenomenon.