Protecting or promoting family unity is a primary focus of families affected by migration. Though national governments differ in their laws and policies in this area–in the income thresholds they require for sponsoring heads of household, in the definitions of family they apply, in the housing requirements they stipulate for dependents–certain overarching principles, and challenges associated with enforcing them, recur across jurisdictions. Closely connected to the wide diffusion of the family unity principle in migration law and policy is a pervasive challenge: establishing the requisite proof that the persons asserting a family relationship are related as claimed. A challenge to the protection of migrant families arises from the fact that state policies and priorities relating to the family unity principle are, like so much else in the migration context, highly susceptible to political changes in national goals and the priorities related to immigration.