Heterocyclic Mesomeric Betaines and Mesoionic Compounds, Volume 137 in the Advances in Heterocyclic Chemistry series, highlights new advances in the field, with this new volume presenting interesting chapters on a variety of topics, including Heterocyclic Mesomeric Betaines, Type A Mesoionic Compounds (1980-2020), Type B Mesoionic Compounds (1980-2020), Recent Developments in the Chemistry of Heteroporphyrins, Carbaporphyrins and Related Systems, Heterocyclic Zwitterions Based on Coupled Polymethines, Meso-ionic Compounds reproduced from Adv. Heterocycl. Chem. 1976, 19, 1-122., and Meso-ionic Heterocycles (1976-1980) reproduced from Tetrahedron, 1982, 38, 2965-3011.