The fourth chapter reconstructs the population history of the Nahua community in Xochimilco for the entirety of the colonial period. Having established the timing, rate, and extent of demographic change, the chapter traces the implications of population decline for social relations. The chapter argues that epidemics and subsequent interventions by the government in the tribute system, incomplete and unsuccessful though they were, represented an assertion of royal authority, one that provided opportunities for Nahua nobles and commoners to contest and renegotiate their relationships with each other and with the colonial administration. These changes proved to be especially threatening to the nobility. In response, the dynastic rulers sought to reassert their own political power within the altepetl, although their success owed much to the efforts of noblewomen in securing and harnessing economic assets to their families’ advantage. They also contracted valuable strategic marriage alliances that bolstered, if only temporarily, their families’ positions in the face of so much loss of life.