Publisher Summary This chapter reviews a specific reaction or property from the heterocyclic point of view––tautomerism. The prototropic tautomerism of heteroaromatic compounds with five-membered rings and one heteroatom is discussed. The most important potentially tautomeric thiophenes and furans are those carrying hydroxyl, mercapto, and amino groups. The tautomeric forms––called pyrrolenines—have been postulated for pyrrole. Heterocyclic compounds carrying hydroxyl groups may be compared with phenols. The tautomeric behavior of phenols is described in the chapter. Both tautomeric forms of polycyclic compounds such as naphthols can be isolated. The potential hydroxyl group includes compounds such as hydroxyfurans, potential dihydroxyfurans, monohydroxyindoles, and α-hydroxythiophenes. Compounds with potential amino groups such as aminofurans, aminothiophenes, aminopyrroles, and aminoindoles are more stable. The chapter includes a discussion of potential methyl group compounds and other substituted pyrroles such as vinylpyrroles and nitrosopyrroles.