Despite many flaws, including variable quality and a lack of universal standards, peer review – the formal process of critically assessing knowledge claims prior to publication – remains a bedrock norm of science. It therefore also underlies the scientific authority of the IPCC. Most literature used in IPCC assessments has already been peer reviewed by scientific journals. IPCC assessments are themselves reviewed at multiple stages of composition, first by Lead Authors, then by scientific experts and non-governmental organisations outside the IPCC, and finally by government representatives. Over time, assessment review has become increasingly inclusive and transparent: anyone who claims expertise may participate in review, and all comments and responses are published after the assessment cycle concludes. IPCC authors are required to respond to all comments. The IPCC review process is the most extensive, open, and inclusive in the history of science. Challenges include how to manage a huge and ever-increasing number of review comments, and how to deal responsibly with review comments that dispute the fundamental framing of major issues.