This chapter examines the influence of partisan and interest group politics on the ratification of International Environmental Agreements (IEAs) and International Labour Organization Conventions (ILOCs). Using an event history analysis and multiple model discrimination tests, the results confirm that IEA ratification is associated with parties’ environmental policy preferences rather than their left–right orientations, with the legislative median providing the best explanation. In contrast, ILOC ratification follows a partisan left–right dynamic, with ministerial autonomy emerging as the best performing model. The chapter also demonstrates that domestic interest groups play a crucial role in ratification processes, with environmental nongovernmental organisations (ENGOs) promoting IEA ratification and trade unions increasing the likelihood of ILOC ratification. Industry lobbies do not appear to systematically block IEA ratification, whereas employer lobbies negatively influence ILOC ratification. Some conditional lobbying effects are observed, including a stronger ENGO effect in more industrialised economies. The results for the control variables reveal that urbanisation, EU membership, peer pressure, and membership in international organisations facilitate IEA ratification, whereas more veto points hinder it. ILOC ratification is positively related to corporatism, peer pressure and international organisation membership but negatively affected by high unemployment rates. The results underline the relevance of both domestic and international factors in shaping treaty ratification behaviours.