Alexander Gabriel emphasizes the necessity of engaging not only in the preparation but also in the voluntary auditing of GRI sustainability reports. Initially, the essential normative framework conditions are presented. Subsequently, the topic is analyzed from the perspective of theoretical approaches, and findings from existing empirical studies are systematized. As part of an original empirical investigation, the value relevance of audited versus unaudited GRI sustainability reports is examined in a European sample. Additionally, other influencing factors such as the financial and economic crisis, as well as cultural aspects, are included in the study.