The debate on ideal theory focuses mainly on whether it can provide a long-term target and a metric for assessing the justice of different institutional arrangements in non-ideal theory. Both critics and defenders of ideal theory typically overlook the role it plays in a model of fairness that can restrict the range of permissible arrangements under non-ideal conditions. In this paper, I explain ideal theory’s fairness role and its part in ensuring an institutional structure that benefits everyone in a society. Critics of ideal theory therefore cannot reject it as simply useless. But I consider how they can question the attractiveness of an ideal-theory-based fairness constraint on non-ideal theory.