Migration—people moving between locations—is now driving much of the demographic change occurring in the United States. Over time, the ebb and flow of migration alters the size, age, and sex composition of local populations. The propensity to migrate varies by age, with young adults the most likely to migrate. Here we summarize new research using recently developed county-level age-specific net migration estimates that identify distinct migration signatures for urban and rural counties. Signatures are unique age-specific net migration patterns that can be used to classify county types. The data provide evidence of spatial clustering in the age-specificmigration patterns in large geographically contiguous areas, such that migration patterns are changing the age structure of entire regions. Such migration patterns have important implications for people, institutions, and communities of both rural and urban America, as well as for the design of policies and practices that foster the development of sustainable communities.