Many scholars have struggled to try to figure out ways to preserve a unitary perspective on international law in light of, and more frequently going beyond, the conservative guidelines formulated by Martti Koskenniemi and his ILC Working Group, collected in the 2006 Report on Fragmentation.1 The proliferation of normative regimes arguably poses a threat to international law's very structure, in the absence of stabilizing elements such as a central legislature, judicial bodies with compulsory and general jurisdiction, and governance legitimized by democratic procedures rather than episodic state consent. It comes as no surprise, therefore, that academics have so far focused on the clash between international law sub-systems, in which norms belonging to different regimes "point at different directions" and leave states with the unenviable choice of complying with one of them while at the same time incurring state responsibility for breach of the others. The editors of the book under review adopt a new approach: Tomer Broude and Yuval Shany focus on equivalent norms, i.e., norms featuring a certain degree of similarity in their content. The working definition of this novel doctrinal category of Multi-Sourced Equivalent Norms (MSENs), set out in the opening chapter, is as follows: Two or more norms which are (1) binding upon the same international legal subjects; (2) similar or identical in their normative content; and (3) have been established through different international instruments or "legislative" procedures or are applicable in different substantive areas of the law [at 5]. Could these norms be the ideal lens for the study of international regimes' interplay?. MSENs provide states and individuals with the possibility or at least the temptation to indulge in regime-shifting:2 given the substantial equivalence of two norms, actors will choose to rely on the one belonging to the "friendlier" regime (in terms of judicial ...