International treaties play an important role in addressing many cross-cutting global challenges, such as sustainable development and climate change. However, why do some states ratify more international treaties than other states do? Studies have highlighted domestic, international, and treaty-level explanations for different ratification behaviours but have paid limited attention to the role of domestic actors, particularly political parties and interest groups, including an identification of the most influential partisan actors. Moreover, virtually no study has systematically compared ratification patterns and their determinants across policy areas, despite the fact that addressing the above challenges requires policy action and integration across a range of areas, including environmental, social and labour policies. This study addresses these shortcomings. This introductory chapter provides the context for the theoretical framework and the empirical analysis, which covers the ratification behaviours of 21 industrialised democracies towards 83 International Environmental Agreements (IEAs) and 150 International Labour Organization Conventions (ILOCs) adopted between 1919 and 2020. With this comprehensive empirical base, the book not only provides novel insights into patterns of ratification behaviour and partisan and interest group effects in international politics but also contributes to the ongoing debate on the importance of domestic and international sources of international commitment.