This chapter offers a critical exploration of the Guiding Principles on Shared Responsibility (GPSR), highlighting their substantive divergences from the established framework of the International Law Commission’s articles on the international responsibility of states and international organizations. It pursues three key aims: first, to introduce the GPSR and identify their principal legal innovations; second, to uncover the rationale behind the drafters’ decision to introduce these changes; and third, to assess whether and how the GPSR address the growing reality of multi-actor involvement in refoulements. Through four structured sections, the chapter dissects the GPSR’s definition of shared responsibility, traces the evolution of its core concepts, and problematizes the role of wrongfulness and causation as bases of international responsibility within this framework. In doing so, it sheds light to the complex framework of shared responsibility in international law while also underscoring its dynamic development. The chapter concludes by proposing a provisionally revised formulation of the GPSR, which will serve as the analytical foundation for examining concrete scenarios of multi-actor involvement in refoulements in Chap. 3.