The main objective of this study is to understand the value of environmental, social, and governance (ESG) disclosure assurance in the context of the development of sustainable finance standards and laws. This study is based on an analysis of 188 comment letters submitted by such actors in the context of public consultations on the development of three new sustainable finance initiatives (the CFA Institute, the Financial Conduct Authority in the UK, and the New Zealand parliament). The study shows these actors' nuanced and often quite critical perceptions of the effectiveness of external assurance in preventing greenwashing and their reservations about its mandatory nature. These actors have raised various criticisms, including concerns about the vagueness surrounding verification practices; the lack of expertise available to conduct assurance in a new, specialized, and complex field; the costs of the assurance process, particularly for small players; and the lack of control over the reliability of the ESG data used. This article contributes to several emerging trends in the literature—in particular, research on governance practices to prevent greenwashing, on the institutionalization of sustainable finance standards and laws, and on the role of rational myths in the assurance process for ESG disclosures.