This chapter traces the reconstruction of boundaries between ethnic groups in the lowlands of today’s Eryuan county from the early Ming by examining their settlement history, military rank, household registration and occupation. The author argues that the integration of military households into civilian lijia units for the purposes of tax collection and labour service assignments from the late sixteenth century led to the formation of Bai communities during the seventeenth and eighteenth centuries; these Bai communities formed the basis for the recognition of the Bai as an ethnic group in the mid-twentieth century. Marshalling empirical evidence from three valley basin societies, the author shows that the Ming transformation in western Yunnan did not result in the formation of fixed, stable ethnic identities, but created fluid social boundaries and ethnic identities. It was this fluidity that eventually led to the formation of new communities based on common property held under the name of village temples and managed by the gentry and village leaders from the seventeenth century onward. The author concludes that these communities were not shaped by language and custom, but emerged through the agency of the local elite, who managed wet-rice irrigation facilities and religious activities through village temples.