“Chihuahua culture” refers to two prehistoric periods of ceramic agricultural occupation in northwestern Mexico. It has long been known that the Medio period (ca. A.D. 1200–1450), with substantial adobe pueblo villages and towns, and ceramics that included elaborate polychrome wares, occurred over a vast region in western and northern Chihuahua and northeastern Sonora. It was also recognized that the preceding Viejo period, with pithouse and wattle-and-daub surface architecture, and less elaborate ceramics, was ancestral, at least in part. However, the geographical extent, dating, and nature of the Viejo-Medio transition were unclear. Recent research by several field projects has demonstrated that Viejo and Medio period occupations were geographically coextensive, as was the transition. A foundation for Viejo period chronometric dating (ca. A.D. 700–1200) is now established by 30 radiocarbon determinations from excavations in west central Chihuahua and one from northeastern Sonora. We discuss the dimensions of the Viejo period in west central Chihuahua with data from 1998–2000 excavations by the PAC (Proyecto Arqueológico Chihuahua). It provides an example of a broad-based transition from a relatively simple to a relatively complex pattern, and new perspectives on the interpretation of radiocarbon dating.”