The freedom to provide services is one of the four fundamental freedoms of the European Union (EU) internal market. Interstate trade in services is impeded by different obstacles consisting of the high level national regulation. The objective of this paper is to reveal the nature of prohibited restrictive national measures. The research is based on the analysis of the Treaty of Rome (now the Treaty on the functioning of the European Union), secondary EU law, the evolution of EU case-law, and a range of doctrinal views. It is argued that the definition of prohibited restrictions is the most complex aspect of the case-law on services. The research compares the concept of the restrictions that are to be abolished within the scope of the freedom to provide services and of other freedoms. This study also investigates the correlation between EU and World Trade Organization (WTO) legal mechanisms in the sphere of the provision of services.