The article analyzes the meaning and significance of the phrase “common constitutional traditions”, which is employed by the Treaty on the European Union. It does so in three ways. First, it explores the historical background, by considering the debate concerning commonality and diversity in Europe, particularly with regard to the thoughts of Montaigne and Montesquieu. As a second step the article discusses two distinctive views of legal traditions, that is to say static and dynamic views. Thirdly, the article discusses the importance of two traditions that show the interaction between the various systems of public law. One is an already recognized constitutional tradition, which is designated by the maxim audi alteram partem. Another is a tradition in the process of being recognized, which is designated by the maxim nemo tenetur se detegere or, alternatively, with the phrase “privilege against self-incrimination”.