Since 1972, numerous large and giant fields have been discovered in the Reforma area of Chiapas and Tabasco States, southern Mexico, and on the offshore Campeche shelf west of Campeche State. The huge carbonate bank with which these discoveries are associated is called the Great Carbonate Bank of Yucatán. Most of the Reforma fields produce from bank‐edge talus, now largely dolomitized, of Late‐Jurassic and Early‐ to Middle‐Cretaceous ages, and drilling depths to the tops of the reservoirs generally range from 3,800‐4,500m. The offshore discoveries include fields which are productive mainly in bank‐edge talus of Paleocene and Late‐Jurassic ages, and depths to the top of production generally are shallower than onshore‐1,260‐3,500m. The petroleum source materials for the Jurassic, Cretaceous, and Paleocene fields are believed to be mainly in Late‐Jurassic strate, but some Early‐Cretaceous sources cannot be eliminated. Middle‐Cretaceous rocks have not yet been identified beneath the offshore shelf from Tampico (Arenque) to Samaria (A. J. Bermúdez) and Conduacán. Although most of the discovered fields contain mainly oil, some of the fields contain gas and condensate.Some porosity and permeability is primary, and most is secondary‐the result of solution, dolomitization, and intense microfracturing. The original trap for the Late Jurassic‐Middle Creataceous fields appears to have been stratigraphic, but the present traps are mainly fractured and faulted domal salt pillows created during the Laramide orogeny.The basis for the discovery of the fields was the widespread presence on the Yucatán Peninsula, and in the States of Campeche, Chiapas, and Tabasco, of Cretaceous through Tertiary backreef or lagoonal facies‐carbonates, anhydrite, and some halite, The drilling of the Pichucalco well in 1971 for the first time showed that reef and/or bank‐edge facies were present. In addition, more than 200 oil seeps were known in a linear zone along the foot of the Sierra de Chiapas, south of and adjacent to the coastal plain. If one assumes a facies distribution similar to that of the Golden Lane, it can be concluded that an extensive bank‐edge talus deposit should lie gulfward from the lagoonal facies. With this geologic concept in mind, seismic work was commenced, and drilling during 1971–1972 led to the dual discoveries of the Sition Grande and Cactus oil fields in 1972. The first offshore discovery, Chac, was made in 1976.The Great Carbonate Bank of Yucatán is believed to include not just the Yucatán Peninsula, but also a part of coastal Veracruz State, where several discoveries have been made in carbonate rocks of Early‐ to Middle‐Cretaceous ages in thrust sheets along the western margin of the Veracruz basin, which are now buried beneath the coastal plain. It is probable that large, subthrust, anticlinal structures underlie the thrust sheets along the western margin of the Veracruz basin. These, when drilled, ma