This paper examines the expansion of the OECD's Programme for International Student Assessment (PISA) and associated growth in the influence of the OECD's education work. PISA has become one of the OECD's most successful "products" and has both strengthened the role of the Directorate for Education within the organization and enhanced the significance of the organization in education globally. We provide an overview of the OECD, including organizational changes in response to globalization and the changing place of the Directorate for Education within the organization, particularly with the development of PISA in the late 1990s. We show how the OECD is expanding PISA by broadening the scope of what is measured; increasing the "scale" of the assessment to cover more countries, systems and schools; and enhancing its "explanatory power" to provide policy-makers with better information. The OECD has also developed the Programme for International Assessment of Adult Competencies (PIAAC) and PISA-based Tests for Schools, which draw on the PISA template to extend the influence of its education work to new sites. The paper draws on data from 33 interviews with past and present personnel from the OECD, the International Association for the Evaluation of Educational Achievement (IEA) and the English and Australian education systems, as well as analysis of relevant OECD documents. We argue that PISA, and the OECD's education work more broadly, has facilitated new "epistemological" and "infrastructural" modes of global governance for the OECD in education.