Parliamentary opposition is certainly an evidence of parliamentarism, but it is not easily defined because of its extreme institutional and behavioural variability, to such an extent that constitutional law fails to be realized. This opposition is partially unwritten as its foundation and practice have preceded its the official recognition by strict law. But, by definition, formal law cannot cover all the oppositional phenomenon’s configuration possibilities, as proven by the importance of conventions beyond written law. Despite everything, its institutionalization was finally required. The vocation of constitutional law is indeed to fix fulcrums for political game, but the question of the juridicity of such a phenomenon must be dealt with. The aim of this work consists of underlining the delicate connection between constitutional law and the parliamentary opposition in Germany and France. If law struggles (imperfectly) to capture it, then in return the opposition mobilizes the juridical instruments that are placed at its disposition. However, this relationship is ambiguous, since constitutional law does not limit itself to writing and because some determinant variables have influenced the implementation of juridical texts. Thus the “performance” of the oppositional phenomenon is not assured, even if it was recognized by the law. The examination of the legal significance of the opposing minority tends to reveal the intrinsic tensions of constitutional law and its political character. Far from being purely static, normativy seems to conceal a certain dynamic.Keywords : parliamentary opposition, system of parliamentary government, parliamentarism, constitutional law, parliamentary law, political law, constitutional order.