Throughout the years, various legal concepts have emerged that recognize that more than two states (in the form of one duty-bearer and one right-holder) may be involved when it comes to the performance of an international obligation, and that this can have relevant implications for treaty and responsibility relations. In this respect one may think of concepts such as obligations erga omnes, interdependent obligations and jus cogens. This chapter explores and analyses the way in which the international law of obligations, through these concepts, has addressed legal relations that involve more than simply one duty-bearer and one right-holder. By doing so, the chapter aims to illustrate the limits but also the B1:L9