This policy brief reports findings from the first stage of the research project Mobile Welfare in a Transnational Europe: An Analysis of Portability Regimes of Social Security Rights (TRANSWEL). It focuses on comparing European Union (EU) migrants’ access to and portability of social security rights in four country-pair case studies. The portability of social security rights within the EU is a key element of free movement of EU workers and other citizens. It involves the recognition and transfer of social entitlements such as unemployment benefit, family benefits, pensions and health insurance, between migrants’ countries of origin and their receiving countries.