The European social security law, i.e. the European regulations on the coordination of social security for migrant workers, has been evolving for over 60 years. This book is the result of a dialogue between academia and practitioners in Germany and Greece on practical issues regarding the implementation of the coordination regulations. On the one hand, it reviews the recent case law of the European Court of Justice (ECJ) on the interpretation of EU social security law. On the other hand, the book provides accounts of how specific issues are resolved by and between national social security institutions. It explores the exchange of experiences of various administrative authorities, both within and outside the reflection of the ECJ's existing case law. Furthermore, the discussion on transnational social security rights in the EU and on issues such as the posting of workers or platform work highlight the existing and future-practical-challenges in this field.