Ancient Mesoamerica was a land of cities (Fig. 1.1). Above all, it was the number and the density of cities that distinguished Mesoamerica from the complex societies in neighboring areas of North America and lower Central America. Further, although ancient Mesoamerican cities interacted to varying degrees with those cultures to the north and south, they interacted most intensively with one another. It was the shared cultural practices produced by those relationships that define Mesoamerica (Kirchoff 1943; R. Joyce 2004a). In short, Mesoamerica is defined by its cities, their interactions with one another, and the cultural patterns created and sustained by those relationships. It is the beginnings of those cities, and their interactions, that form the principal themes of this volume.