The concept of critical theory is ambiguous. It combines in a productive way two meanings of the word Kritik which were developed in classical German philosophy. The one meaning stems from the Kantian programme for a transcendental philosophy and signifies the testing of legitimacy. The other meaning goes back to the Young Hegelians' attitude to the opposition of theory and practice and signifies negation. Ever since Marx developed his concept of a realist science, which sought to draw the consequences from the faltering beginnings and eventual failure of German idealism, the two meanings of the concept of Kritik have been fused together. Thus the ambiguity of the concept has been passed on from generation to generation and can be traced even to present-day neo-Marxism. This is what I wish to demonstrate in what follows.