European Union (EU) negotiations take place within the framework of a set of formal and informal institutions. Member states negotiate in the organizational context of the Council and the European Council, take decisions through alternative procedures and decision rules, and sustain a set of norms for the conduct of negotiations. While the effects of these institutions on negotiation behaviour and outcomes have received extensive attention, the question of why these particular institutions have been established or evolved remains underexplored. This contribution makes an argument in favour of systematic attention to the design of negotiation institutions in the EU, and suggests that we should draw on general theoretical approaches to institutional design for these purposes. Moreover, rather than engaging in the exercise of trying to prove or falsify theoretical approaches, we should exploit the potential for explanatory complementarities in concrete empirical domains. To this end, the paper advances a domain-of-application approach to institutional design and illustrates its applicability to EU negotiation institutions through four empirical illustrations.