Abstract
This chapter focuses on the annular tautomerism of heterocyclic six-membered rings. The most important change due to the tautomerism of heterocycles is the explosive development of the computational methods and studies arising from the computer science revolution. The level of calculations became high enough to ensure precision exceeding that of many experimental results. Recent calculations of equilibrium constants include solvent effects with different approximations as well as other refinements such as zero-point energy correction. Many new experimental techniques have also been applied to the studies of tautomerism such as homonuclear and heteronuclear coupling constant determinations and photoelectron, fluorescence, and ion cyclotron spectroscopy. Several azine cations are also discussed in the chapter.