Whether or not a certain norm is legally binding upon international actors may often depend on whether or not the instrument which contains the norm is to be regarded as a treaty. In this study, the author argues that instruments which contain commitments are, "ex" "hypothesi," treaties. In doing so, he challenges popular notions proclaiming the existence of morally and politically binding agreements and so-called soft law'. Such notions, Klabbers argues, are internally inconsistent and founded upon untenable presumptions. Moreover, they find little support in the pertinent decisions of municipal and international courts and tribunals. The book addresses issues of importance not only for academics working in international law, constitutional law and political science, but also for practitioners involved in the making, implementation and enforcement of international agreements.