Throughout this book, the focus has been on developing a concept that captures the practical phenomenon of sharing international obligations, proceeding from the premise that the performance of an international obligation is not always up to only one state or international organization, and that this can have relevant legal implications. In Chapter 1, it was discussed how the idea of shared obligations has been recognized in both scholarship and practice, but comprehensive conceptualization of the notion lags behind. This was placed in the context of an overall tendency to avoid engaging with primary obligations at a more systematic level in the international law of obligations, influenced at least in part by a few fundamental choices made by the ILC during its work on the codification and progressive development of the law of treaties and the law of state responsibility. In particular, the decision to steer the focus of its state responsibility project away from primary rules that define the content of international obligations towards secondary rules1 seems to have left us with the impression that distinguishing between different categories of obligations is normally not relevant in the international law of obligations.