The book examines the politics of knowledge in global social policy, investigating how international organisations (IOs) have contributed to the emergence and development of social security as a global policy field. It reconstructs the role of numerical knowledge in the International Labour Organization (ILO) and the World Bank, theorising how IOs contribute to epistemic infrastructures of global social security. The book shows how IOs’ knowledge production has led to a continuous refinement of the meaning and purpose of social security. First, it reveals how IOs arrived at a shared conception of social security: what the book calls an ontological framework. Second, it traces how numbers have increasingly enabled the assessment of countries according to shared benchmarks: what the book calls an evaluative framework. The author demonstrates the political and epistemic work involved in universalising knowledge of social security, while highlighting the limits of governing by numbers in global social policy.