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As the TTIP negotiations with the US continue, Europe is hardening its immigration policy. It is estimated that constraints imposed on the mobility of workers will cost up to several hundred billion euros, a huge number compared with the potential gains expected from the TTIP. The aim of this paper is to review the studies that have quantified the gains from liberalizing migration, and to discuss their underlying assumptions and mechanisms. If one assumes that liberalizing migration leads to a convergence of wages, world production could increase by 50-150%, according to the first studies on the issue. However, more recent work, which takes migratory preferences into account, is less optimistic and shows that the volume of potential additional migrants has likely been overestimated. Still, the gains from easing migration restrictions are considerable, especially if positive externalities, such as increased educational incentives, are taken into account.