The issue of worker mobility is intrinsically tied to the European Union, whose project is to create a single European market. This is seen as the central concept that will bring Member States and European citizens closer together and create “de facto solidarities” underpinned by legal relations that should be able to prevent any further military conflict in Europe. The underlying economic approach to establishing a European Community explains why the European market must extend to all economic agents, including workers. Since the early 1950s, European institutions have had the objective of promoting the economic and social benefits of worker mobility, which implies encouraging mobility and removing obstacles to it. The development of the European Union in its social and political dimensions, initially not very concerned by the market, involves confronting the economic, institutional, socio-political, and legal challenges related to worker mobility. Among the areas that fall within the EU’s supporting competences – such as sport – worker mobility is particularly relevant, mainly because national issues remain strongly present, but also because significant mobility issues have not yet been regulated within a transnational European institutional framework. These mobility issues include standard contracts for salaried professional athletes, numbers of locally trained players in professional clubs, and the debate on the regulation of diplomas for the supervision of physical and sports activities that give sports educators the legal right to practice their profession in return for remuneration.