The prehistoric cultural sequence of southern and central Arizona in the southwestern United States is characterized by a particularly long span of nomadic small-group hunter-gathering/foraging (11000 BC to 2000 BC); followed by a long period of small-group semimobile agriculture (2000 BC to AD 475); the formation and long developmental sequence of stable sedentary agricultural villages of small to moderate size (AD 475 to AD 1275); rapid aggregation into relatively short-lived towns (AD 1275 to 1300); and, ultimately, abandonments and dissolution into smaller mixed-subsistence semimobile and sedentary settlements (ca. AD 1450). In the scheme of this sequence of 11,000 years or more, there were three dramatic, rapid, gamechanging periods of culture change, two of which involved aggregation and one, disintegration. This chapter focuses on the second of the two intervals of aggregation, with brief reference given to the fi rst.