Dr. Zane U. Segle
I am adding a description of Teotihuacan taken from the internet (credited below):
Teotihuacan arose as a new religious center in the Mexican Highland, around the time of Christ. Although its incipient period (the first two centuries B.C.) is poorly understood, archaeological data show that the next two centuries (Tzacualli to Miccaotli phases; A.D. 1-200) were characterized by monumental construction, during which Teotihuacan quickly became the largest and most populous urban center in the New World. By this time, the city already appears to have expanded to approximately 20 square km, with about 60,000 to 80,000 inhabitants (Millon 1981:221). The development of the city seems to have involved inter-site population movements, exploitation of natural resources, an increase in agricultural production, technological inventions, establishment of trading systems and other kinds of socio-political organizations, and attractive belief systems. By the fourth century, unmistakable influences of Teotihuacan were felt throughout most parts of Mesoamerica. Teotihuacan was the sixth largest city in the world during its period of greatest prosperity, according to an estimated population of 125,000 (Millon 1993:33). The city seems to have functioned for centuries as a well-developed urban center until its rather sudden collapse, possibly in the seventh century. The place was called Teotihuacan by Nahuatl speakers several centuries after the city's fall, but its original name, the language or languages spoken there, and the ethnic groups who built the city are still unknown.
Many surveys, excavations, and studies of materials have been made for more than a century, employing different kinds of approaches and techniques. Since the first scientific inter-disciplinary investigation was carried out by Manuel Gamio in 1917-22, several explorations have revealed specific cultural traits and helped situate Teotihuacan prehistory within the Mesoamerican chronological framework. The Teotihuacan Mapping Project (Millon 1973; Millon et al. 1973) directed by Rene Millon contributed substantially to forming current views of the city. The Settlement Survey Project in the Basin of Mexico, directed by William Sanders, placed Teotihuacan in a regional context (Sanders, Parsons, and Santley 1979). Various explorations in the 1960s and 1970s in residential compounds in the city provided information about social life and categories. Meanwhile, monumental constructions were excavated by Mexican archaeologists from national institutions, most recently the Instituto Nacional de Antropologia e Historia (INAH), since the beginning of this century. Most of buildings now seen in the Archaeological Zone in Teotihuacan were actually those excavated and consolidated by these national projects. The Feathered Serpent Pyramid was among these, first excavated in 1917-22. Before concentrating on the Feathered Serpent Pyramid, you may want to visit other major monuments placing the pyramid in the wider context of the city.
Narrative taken from: http://archaeology.asu.edu/teo/intro/intrteo.htm.