In this PhD dissertation I explain how different forms of international trade institutions affect domestic coalition patterns. Negotiated trade policy instruments create incentives for sector-wide interest representation, while administrative instruments elicit interest aggregation on a more product-specific, intra-sectoral level. I provide a rationale why reciprocal trade negotiations foster sector-wide associations, whereas trade rule enforcement elicits intra-sectoral coalitions. In contrast to existing explanations, these propositions can account for coalition patterns during both the GATT 1947, which consisted of a series of trade negotiation rounds, and the WTO (1995-), which combines negotiations with binding administrative enforcement in the form of international dispute settlement. Comprehensive evidence from EU trade policy lobbying confirms the explanatory force of the hypotheses. Sectoral peak associations co-ordinate interest representation during the GATT and WTO Rounds, whereas private interests predominantly organise on an intra-sectoral level when filing petitions for enforcement instruments administered by the European Commission. Exhaustive databases provide evidence about the predominantly intra-sectoral trade associations that lodge complaints with administrative instruments such as Anti-Dumping, market access investigations and WTO dispute settlement, while other sources reveal how sector-wide peak associations organise during negotiation Rounds. I provide supportive evidence from existing research on interest representation in American trade policy, and control for other factors such as industry concentration and non-trade regulation. The shift from negotiations-only to enforcement is further significant for interest representation in four selected sectors of European industry: chemicals, pharmaceuticals, steel and textiles. Although each of these industries has a long history of sector coherence, they all reorganised their membership and representation structure to accommodate for the increased importance of enforcement instruments. The adoption of direct company membership and/or the inclusion of product-specific trade associations accompany the decrease in importance of the sectorwide peak association in a world where the provision of detailed information for judicial enforcement is starting to weigh more than the political clout of sector-wide peak associations during negotiations.