Until the early seventies, tautomeric i. e. fast and reversible rearrangement reactions accompanied by migrations of carbon-centered groups - were practi- cally unknown. For a long time it was assumed that the family of tautomeric reactions was confined to proto tropic transformations only. However, the discovery in the fifties of the reversible metallotropic rearrangements showed the domain of migratory tautomerism to be substantially broader. The synthesis of the metallotropic compounds was based on the substitution of a proton in prototropic compounds by an organometallic group. This approach rarely proved fruitful when attempting to effect tautomeric rearrangements of organic and organometallic groups formed by the elements to the right of carbon in the Periodic Table. By contrast, a novel approach involving an analysis of the steric requirements inherent in the structure of the transition state of a reactive center and an examination of the stereodynamic possibilities has given rise to a target-oriented molecular design of compounds capable of rapid and reversible intramolecular migration of the type indicated. The implementation of this ap- proach, which is the subject of the present book, has already led to the preparation of new tautomeric compounds in which such heavy organic migrants as acyl, aryl, sulfinyl, phosphoryl, arsinyl, and other groups migrate in molecules at a frequency 6 9 of 10 -10 S-I at ambient temperature, i. e. , at the rates comparable with protonic migrations.