Over the last few years, an increasingly sophisticated literature devoted to normative questions arising out of the enforcement of immigration law had developed. In this essay, I consider what sorts of constraints considerations of justice and legitimacy may place on the enforcement of immigration law, even if we assume that states have significant discretion in setting their own immigration policies, and that open borders are not required by justice. I consider constraints placed on state or national governments, constraints on enforcement by substate governments, and constraints on the actions of individuals. I show that there are significant limits on what states may do and what they may require substate governments and individuals to do, in enforcing their immigration laws, but that these constraints are not clearly incompatible with significant state discretion in setting immigration policy. Nonetheless, consideration of justice in enforcement is necessary for any complete normative account of immigration.