The chapter recharacterises the founding instrument of international organisations as constitutions. They function as a legal basis for the organisation, they contain provisions about the mission of the organisation, about the organs/bodies and their competences, and regulate the relationship between the organisation and those who are legally subjected to it. It traces two waves of theories which have espoused different conceptions of constitution: The first wave revolved around the “small c-constitution” in the more neutral sense. The second wave postulated constitutions “with a capital C” that enshrine the constitutionalist trinity: rule of law, human rights, and democracy. In the current constellation of a global shift of power and ideology, a third theory for constitutions of international organisations, more responsive to the global social question and to the demands of the global south, is emerging. This third theory deserves to be pulled out into the light and should be fleshed out further. It should, on the one hand, not fall back on the small-c constitution and, on the other hand, take on board new principles, notably social transnational solidarity and contestatory democracy. This intellectual contribution can provide a basis for responses to the current pushbacks against international organisations.