This paper shows that three versions of the principle of proportionality exist in international law, and that these three have different constitutional functions. The horizontal version 1 applies, inter alia, in the field of countermeasures including self defence. The principle here refers to the relation between action (breach of international law) of the state and the admissible reaction of another state. The diagonal version 2 concerns the relation between and national public interest and particular interests, of individuals (in human rights law and international humanitarian law) or of investors. The vertical version 3 refers to the relation between a global public interest, for example in free trade, and particular interests of states.