The 2004 and 2007 enlargements of the European Union were unprecedented in a number of economic and policy aspects. This IZA essay provides a broad and in-depth account of the effects of the post-enlargement migration flows on the receiving as well as sending countries in three areas: labour markets, welfare systems, and growth and competitiveness. It concludes that on the whole, no negative immigration-related effects could be detected, that post-enlargement migration contributes to growth prospects of the EU and that restrictions on the free movement of workers bring about difficulties.