[Advances in Heterocyclic Chemistry] Volume 64 || Recent Developments in Ring–Chain Tautomerism I. Intramolecular Reversible Addition Reactions to the C=O Group**This chapter is the first part of a review that updates an earlier work by R. E. Valters and W. Flitsch, Ring-Chain Tairtomerism (A. R. Katritzky, ed.), Plenum Press, New York and London, 1985. The second part of this review, to be published in a subsequent volume of Advances in Heterocyclic Chemistry, includes data (1982‒1993) for intramolecular reversible addition reactions to C=N, C≡N, C=C, and C≡C groups.