Four years of intensive archaeological survey, supplemented by excavations in the Teotihuacán Valley, Mexico, have greatly clarified our picture of the transition from Preclassic to Classic. The definition of two new phases of culture history, Tezoyuca and Patlachique, that belong to the Terminal Preclassic (post Late Ticoman) strongly suggests a local development of the great Classic city of Teotihuacán. This runs counter to the hypothesis that a great migration, possibly from the Gulf Coast region, was responsible for the fully developed Classic Teotihuacán culture. Present evidence indicates that the Patlachique phase is intrusive and appears in the Teotihuacán Valley possibly as the result of migrations from the southern Valley of Mexico, perhaps precipitated by the eruption of the volcano Xitli. When peoples carrying this culture entered the Teotihuacán Valley, they encountered the Tezoyuca culture and gradually absorbed it. The fusion of these two groups eventually produced Tzacualli, which is the first recognizable phase of the Teotihuacán culture.