ESG assurance in the United States We provide the first large-sample evidence on third-party verification of firms’ environmental and social metrics in ESG reports (ESG assurance) in the United States. Focusing on the S&P 500 from 2010–2020, we document a striking increase in not only the number of firms with ESG assurance—in 2020 (2010), 76% (38%) of the S&P 500 had an ESG report and 46% (16%) involved assurance—but also the number of metrics assured. Unlike financial audits, ESG assurance var...