Social security coordination is a central pillar of the European Union. When the European Economic Community was gaining momentum in 1958, the creation of a system of co-ordination of social security became necessary. Until today, the Treaty Regulations No. 3 and 4 have outlined this system, giving evidence of the utmost importance that the Community has attributed to the question of social security coordination. The diversity of social security schemes in the Member States made of coordination an intricate task. Coordination law had to offer answers to migrations every single problem in other member states, no matter if it was for professional or private (tourist) reasons. At the right time, the new Commentary reflects the political relevance of social security coordination law within Europe-wide debates about immigration caused by the new Member States and the given social benefits to their immigrants. Article by article the new commentary deals with the coordination law as it is laid down in Regulation (EC) No. 883/2004 and its Implementing Regulation (EC) NO. 987/2009. In addition, it encompasses accompanying legislation such as Regulation (EEC) No. 7/79 and extends to social security coordination established in specific agreements between the EU and non-EU countries like Switzerland and Turkey. A further part of the Commentary deals with the decisions of the Administrative Commission that play an important role in the application of these regulations. Special emphasis is placed on the overarching case law of the Court of Justice of the EU. Important legal literature has been taken into account in order to underpin the functioning of social security coordination and its contribution to offering solutions. The authors of the Commentary are an international team consisting of university professors, a former member of the European Commission, a high-ranking civil servant in the German Ministry of Labour and Social Affairs and a presiding judge of a high German Social Court. They have all been active in the field of social security coordination for more than 30 years.