This article examines limitations on the social rights of mobile EU workers in EU law through the national definitions of ‘worker’ and ‘socially insured’ person. It analyses EU laws and judgments of the CJEU and provides a comparative analysis of the core elements of the concept of ‘worker’ and ‘insured person’ in national laws and potential advantages for mobile EU workers of their broad interpretation. We argue that the present narrowly interpreted terms overwhelmingly put mobile EU workers at a disadvantage, which might lead to indirect discrimination and non-compliance with the principle of proportionality, which are prohibited in EU law with regard to free movement. Mobile EU workers are hindered in becoming ‘workers’ and ‘insured persons’, but targeted collection and processing of statistical data could help them to assert their rights and to enjoy social entitlements, and to avoid being pressurised to accept poor wages and working conditions.