This chapter clarifies some key terms and situates the phenomenon of sharing international obligations within the current body of international legal doctrine. The conceptualization of shared obligations takes place in the context of the international law of obligations, and it is first discussed what is meant when reference is made to this body of law. The focus then turns to the concept of shared responsibility in international law, and some preliminary reflections are offered on how the concept of shared obligations contributes to the ongoing discussion on problems of shared responsibility in legal doctrine. It is then discussed how the idea of sharing international obligations has been recognized in international legal literature and proceedings before international courts, reflecting an (at times implicit) assumption that the sharing of international obligations has relevant legal implications. Nevertheless, the notion of shared obligations remains conceptually underdeveloped. Finally, it is set out how this lack of conceptualization of shared obligations is not a result of the notion’s irrelevance but may be explained – at least in part – by a few fundamental choices that have been made during the ILC’s project of codification and progressive development of the international law of obligations.

The analysis in this chapter proceeds from the assumption that shared responsibility for a breach of a shared obligation has been established and focuses on the content of that responsibility. In the case that secondary obligations of reparation or cessation arise, what can be claimed from which responsible actor, and to what extent is this influenced by the fact that the underlying primary obligation is shared? And if injured parties wish to effectuate these secondary obligations through international adjudication, are there any obstacles to the claiming of cessation and reparation? It will ultimately be argued that conceiving of the content of shared responsibility as consisting of shared obligations incumbent on all responsible actors can help untangle which of the responsible states or international organizations is bound to do what in terms of cessation and reparation. In this context, the distinction between divisible and indivisible shared obligations has an important role to play. After all, determining whether responsible actors are each bound only to their own share of cessation or reparation or whether, alternatively, all of them become bound to achieve a common result has direct implications for what can be claimed from whom.