This chapter establishes the theoretical framework of the book, focusing on the influence of partisan politics and interest groups in shaping ratification outcomes. It argues for a nonstate-centric perspective on international cooperation, emphasising the role of national governments and domestic actors. The chapter explains the impact of party competition and electoral politics—particularly through environmental and left–right ideological lenses—on the ratification of International Environmental Agreements (IEAs) and International Labour Organization Conventions (ILOCs). Five models of partisan effects are introduced: the weighted compromise, the legislative median, veto players, ministerial autonomy, and the head of government. This material is followed by an explanation of interest group dynamics, in which environmental nongovernmental organisations (ENGOs) and trade unions advocate for ratification, whereas industry and employer lobbies oppose it. The chapter concludes with the formulation of several hypotheses, setting the stage for the next chapter, which details the methodology for testing them.