The transfer system in football is one of the most regulated aspects of sports law and this thesis shall focus on some of its specificities, such as the registration of players and the need for registration periods. There are several restrictions on the registration of players with the objective to prevent a distortion of the footballing competitions, also referred to as the sporting integrity of competitions, and to provide a level playing field, that are justified by a concept known as the specificity of sport. This thesis shall illustrate these restrictions and its justification, in particular, the restrictions of fundamental rights such as the freedom of movement and the freedom to work, by analysing the cases submitted to the European Court of Justice and the Court of Arbitration for Sports.