The UN system requires far-reaching changes so as to achieve the objectives of the UN Charter (e.g. with regard to human rights and maintenance of peace) more effectively. European integration law suggests that ‘international constitutionalism’ offers the most effective approach for strengthening the rule of law and peaceful cooperation among democracies. Section 2 outlines basic principles for a constitutional theory of international law. Section 3 discusses the difficulties of ‘constitutionalizing’ the state-centered and power-oriented concepts of the UN Charter. Section 4 explains why the successful Uruguay Round strategy for replacing the old GATT 1947 by the new World Trade Organization (WTO) – notably the ‘package deal negotiations’, the incorporation of other worldwide treaties into WTO law and the mandatory WTO dispute settlement and enforcement systems – offer important lessons for the needed reforms of the UN Charter.