For most of the 20th century, following the Second World War, non-discrimination and multilateralism were core pillars of the international, rules-based trade system. From the 1990s onwards, however, the number of (discriminatory) bilateral and regional trade agreements jumped sharply. The World Trade Organization (WTO), and its predecessor the General Agreement on Tariffs and Trade, permit the negotiation of such arrangements under strict, albeit loosely enforced, conditions. As these regional and bilateral agreements have proliferated, so have questions about their impact on the multilateral trade system. Are these alternative agreements building blocks that allow trade liberalization to continue in a period during which multilateral negotiations have been mostly stagnant? Can regional trade agreements (RTAs) lead the way on deeper integration and lay the foundation for new rules in new areas at the WTO? Or will RTAs prove to be a substitute, rather than a complement, leading to increased discrimination and undermining the multilateral trade system?