This chapter provides the first comprehensive review covering all main aspects of the chemistry of iodine heterocycles including preparation, structure, and synthetic applications. The most important and practically useful polyvalent iodine heterocycles have a five-membered ring, although several examples of three-membered, four-membered, and six-membered heterocycles with an iodine(III) or iodine(V) atom in the ring are also known. In general, iodine is not capable of forming conjugated cyclic systems with aromatic stabilization because of the large atom size and the semi-ionic nature of the hypervalent I-C, I-N, and I-O bonds. Despite the lack of aromatic conjugation, the five-membered heterocyclic iodine compounds (benziodoxoles and benziododazoles) have a considerably higher thermal stability compared to their acyclic analogs, which has made possible the preparation and isolation of iodine-substituted derivatives, such as bromobenziodoxoles, tosyloxybenziodoxoles, alkylperoxybenziodoxoles, azidobenziodoxoles, cyanobenziodoxoles, amidobenziodoxoles, alkynylbenziodoxoles, and trifluoromethylbenziodoxoles. Derivatives of benziodoxoles have found wide practical applications as oxidants and reagents for atom-transfer reactions in organic synthesis.