The principle of primacy of EU law is one of the oldest cornerstones of European integration. While the European Court of Justice has always stressed its unconditional character, Member states have developed different approaches over time, seeking in one way or another to mitigate the limitation of their sovereignty. The question of the precise definition of this relationship between the two legal sources, especially concerning national constitutional law, has often been dealt by the national constitutional courts in the past 40 years. The present thesis has the ambition to illustrate the evolution of the actual situation in four selected countries, including Germany, Poland, Austria, and France. The focus lies on the judicial reaction concerning the primacy principle which is why several court cases are analyzed. Furthermore, the historical development of the principle is outlined, allowing us to understand its origins as well as its limitations.