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Molecular rearrangements of five‐membered ring heterocycles are conveniently classified in five categories based on the number of participating side‐chain atoms. These include the rearrangements which operate by a ring contraction‐ring expansion mechanism, the Dimroth rearrangement, the Cornforth rearrangement, the Boulton‐Katritzky scheme, the bond‐switch rearrangements with sulfur as the pivot atom, and many other isolated cases. A concise description of these rearrangements in the monocyclic series is given with particular emphasis on the thermal reactions. The photoisomerizations have been included only when necessary for discussion, since an excellent recent review exists on this topic (ref [9]).