AbstractThis chapter analyzes the evolution of the Intergovernmental Panel on Climate Change (IPCC) from a specialist organization of climate scientists into an institution at the nexus of science and politics. We explain how the IPCC became the primary scientific authority for policymakers, the public, and climate activists on the existence, severity, consequences of, and, increasingly, possible solutions to anthropogenic climate change. We assess its influence on policymakers and governments, while examining the various tensions, critiques, and contradictions that the organization and its leaders have had to grapple with across its 32-year history, during which it successfully developed a distinct identity as a trusted provider of comprehensive scientific assessments. Our analysis also focuses on the institutional reforms that helped restore legitimacy to IPCC after ‘climategate’ and other controversies.