This thesis investigates international organizations’ (IOs) efforts at constructing an epistemic infrastructure within social security policy, as a constitutive element of the emergence, existence and realization of global social policy. Thereby, it asks, first, how global social (security) policy has been made possible and how it has been realized through knowledge. Second, it scrutinizes the political implications and effects of epistemic infrastructures. The thesis concentrates on the International Labour Organization (ILO) and the World Bank as crucial actors in global social policy-making, which have always been concerned with the production of knowledge as well as with governance. Empirically, the thesis utilizes a Grounded Theory-based analysis that covers 272 documents over the long historical period from 1919 (the ILO’s inception) until 2015. Theoretically, it is supported by Foucaultian discourse analysis, theories of quantification and classification, and theories of knowledge in policy-making. Taken together, the thesis has found that IOs do not only contribute to global social policy by producing standards or regulation but also by observing and describing global fields of policy in seemingly disinterested fashion. These inscriptions of knowledge then enable and shape social policy-making of other actors, by providing an epistemic infrastructure through which the respective field of policy-making can be understood and on the basis of which policy-making can occur. Through the empirical analysis, the thesis has reconstructed how the epistemic infrastructure of global social security policy has, over time, been transformed and embraced new functions. A first challenge in the construction of a shared epistemic space has been the production of a shared ontological framework. Actors have thus first worked towards establishing facts, categories and concepts of social security policy, and forging links between units via comparison and quantification. While the production of such an ontological framework was still contested during the interwar period, it developed more and more into a technical process after WWII. Thus, the progressively taken for granted and established ontological framework slowly began embracing new functions - assessment and evaluation, which allowed for assessing policies through shared normative standards. The thesis thus reconstructs a qualitative change in the function of infrastructures - from description to more active evaluation of performance. Based on the thesis’ research results, it can be concluded that foundational epistemic infrastructures exert implicit but extensive political effects by structuring the space of possibilities of policy-making.