Diversity, equity, and inclusion (DE&I) issues are pivotal for fostering firms sustainable success in contemporary business landscapes. This chapter aims to assess the quality of DE&I disclosures through sustainability reports and pinpoint the factors influencing such disclosures. Three research hypotheses, based on agency theory, were tested using a regression model on a sample of 157 worldwide companies. A DE&I disclosure index was constructed to evaluate disclosure quality, and a content analysis was performed to investigate the sustainability reports. Results showed a positive and significant impact of board gender diversity, board independence, and board size on the quality of DE&I disclosure within sustainability reports. This study broadens the application of agency theory and the analysis of DE&I disclosures by incorporating sustainability reports. Additionally, it deepens our understanding of the factors influencing DE&I disclosures, which has been an area of limited investigation. The findings have significant implications for corporate executives, high-level corporate governance, and policymakers, suggesting that enhancing board gender diversity, board independence, and board size has a significant positive impact on DE&I disclosure.