DURING THE CONVENTION organized by the Mexican Anthropological Society in Tuxtla Gutierrez in 1942, an erroneous theory associating Teotihuacan with the Toltecs was finally cleared up. For many years we had realized that the origins of Teotihuacan's culture were to be found outside the central plateau of Mexico. But it was only after deeper knowledge of the Olmec culture and its links with the development of Pre-Classic Mexican cultures had been gained, and after the identification of Tula in the present State of Hidalgo as one of the Toltec centers, that we could determine a more precise period of occupation for Teotihuacan. Over the past twenty years, research done on the central plateau and the Gulf Coast of Mexico by Armillas, Acosta, Bernal, Michael Coe, Drewitt, Millon, Noguera, Sanders, Sejourne, as well as by teams from the National Institute of Anthropology and History of Mexico, and the U niversity of Rochester, among others, have pushed back the chronology of Teotihuacan's various periods of occupation. Also during these years, we have developed a different image of the urban characteristics of a city. A new chronology worked out by Millon (Millon, 1969) has replaced the one by Vaillant which dates from the 1930's (Vaillant, 1955) and has modified the one by Armillas which dates from the 1950's (Armillas, 1950, 1952). Finally, the surveys made by Millon (Millon, 1960, 1967, 1969) seem to prove definitely that Teotihuacan was a city and not merely a ceremonial center occupied by a small permanent population. Even so, the final studies are still incomplete and excavations concentrated in limited areas. We still have limited knowledge of the relations of Teotihuacan with adjacent regions (Parson, 1968c) and with the rest of Mesoamerica, as well as of its economic base and social structure.