Degenerate ring transformations of heterocycles are classified as reactions in which a heterocyclic system is converted into the same hetercyclic system. This monograph covers an authoritative, comprehensive overview of a host of degenerate ring transformations in five- and six-membered heterocycles. It shows how by the use of 15N-labeled, 13C-labeled or selectively substituted compounds these degenerate ring trnasformations can be discovered and hwo most of the results can be explained by the Addition Nucleophile, Ring Opening, Ring Closure [ANRORC] mechanism. Another main topci of the monograph is the occurance of degenerate ring transformations.