Sustainability champions: the transformative role of internal auditors in ESG assurance – a systematic review and future directions

Purpose This study conducts the first systematic literature review (SLR) on the key role and involvement of internal auditors (IAs) in Environmental, Social and Governance (ESG) assurance. It systematically maps and evaluates the existing research by analysing publication trends across top journals, years, geographical locations, methodologies and theoretical frameworks. Furthermore, it identifies the dominant and emerging themes, such as the extent of IAs' involvement in ESG assurance, their roles, influencing factors and challenges faced. Additionally, the study identified research gaps and suggested a future agenda for advancing existing knowledge and practice implications in this area. Design/methodology/approach The review offers a comprehensive and up-to-date SLR and is conducted by analysing 55 peer-reviewed articles published between 1998 and 2025 in over 20 countries, all published in 33 top-ranked journals according to the Chartered Association of Business Schools (CABS) 2024 ranking. These studies were identified through comprehensive searches conducted in Scopus and Web of Science utilising distinctive search strings and using a range of ESG-related keywords. The identified initial sample of 7,807 articles was filtered and screened by employing the PRISMA framework, using meticulous inclusion and exclusion criteria to produce the final sample, which included studies with diverse methodological approaches, reflecting a broad and balanced perspective on the topic. Findings The findings highlight the significant shift in IAs` roles from focusing on traditional compliance activities to strategic partners in sustainability governance and assurance. The analyses show that their involvement in ESG assurance varies across ESG domains, industries and organisational maturity levels. IAs now serve as assurance providers, consultants and advisors on ESG reporting, risk management and accountability supporters and key contributors to sustainability and environmental strategy. Also, their involvement is shaped by regulatory and market pressures, stakeholder expectations, governance practices and leadership support. Despite the growing importance of IAs` involvement, they face challenges, such as limited ESG expertise, role ambiguity, resource constraints, regulatory inconsistencies and a lack of standardised assurance frameworks. Nonetheless, their effective involvement mitigates greenwashing risks, improves ESG reporting quality and supports strategic decision-making. Research limitations/implications The study acknowledges several limitations, including potential language and publication bias due to the exclusion of non-English publications and grey literature and reliance on peer-reviewed academic sources, which may ignore potential relevant regional studies and limit the comprehensiveness of the review. Additionally, the study does not fully capture emerging regulatory shifts or the regional- and sector-specific variations in ESG assurance practices, which may affect the generalisability of the findings. To address these gaps, the authors suggest future research directions which are critical for strengthening internal audit capabilities and for informing policy, practice and the broader discourse on ESG assurance. Originality/value This SLR offers a novel contribution by shifting the interest of ESG assurance literature from external providers to IAs. It provides a comprehensive and timely analysis of the evolving responsibilities and contributions of IAs in sustainability governance. The study supports audit committees, regulatory authorities and corporate actors seeking to strengthen ESG assurance systems by offering actionable insights and practical recommendations.