This chapter presents the book’s core theoretical contribution of the transnational lawmaking coalition (TLC), a concept that brings fresh insight to the IR and IO literatures. Operating outside the IO borders of the treaty body system, TLCs are informal and temporary coalitions of state-empowered expert bodies and issue professionals that, in pressing for clarification of state obligations through the instrument of treaty interpretations, can contribute to the development of international law without direct government involvement. This chapter distinguishes TLCs from other collective transnational actor-types, introduces the main triggers of a TLC’s genesis, explains the logic behind its members’ participation in such a coalition, and articulates how TLCs organize their drafting work and under which conditions they are influential.