In this chapter, the key concepts to be analysed are social security system, statutory pension scheme, main and supplementary pension protection. The analysis of the said concepts is considered necessary in order to point out the importance of placing a scheme under Coordination Regulation. Specifically, the Regulation 883/2004 on coordination of social security systems does not provide a definition of the term ‘social security systems’ despite the title of this legal instrument and despite the fact it entails a large number of definitions of the concepts used therein. This lack of definitions has an impact on the application of Coordination Regulation trying to answer the question which pension schemes fall under its scope. In parallel, the social role of supplementary pension rights is highlighted. Coordination Regulation tends to provide a broad protection of pension rights including both main and supplementary pension schemes. However, it is the member states that will decide if they bring their supplementary pension schemes under Coordination Regulation utilizing the possibility provided i.e. the right to declarations. The example of French supplementary schemes is referred as schemes placed under Coordination Regulation aiming at their protection from competition law. This is also why the Directive 98/49/EC on safeguarding supplementary pension rights of employed and self-employed persons moving within the Community and the Directive 2014/50/EU on minimum requirements for enhancing worker mobility between Member States by improving the acquisition and preservation of supplementary pension rights are presented, as well as the relationship between Regulation on coordination of social security systems and supplementary pension rights. In the same context, the concept of portability of pension rights is introduced.