Sustainability information has moved from glossy supplementary brochures into regulated annual reports. As it has, investors and regulators have asked a natural question: how reliable is it? The International Auditing and Assurance Standards Board answered with International Standard on Sustainability Assurance (ISSA) 5000, General Requirements for Sustainability Assurance Engagements.
The IAASB approved the standard in September 2024 and published it in November 2024. ISSA 5000 is effective for assurance engagements on sustainability information reported for periods beginning on or after 15 December 2026 — meaning most calendar-year reporters will see it applied to their 2027 reports, with earlier application permitted.
One standard for limited and reasonable assurance
ISSA 5000 is designed as an overarching standard. It covers both limited assurance engagements, which are the norm today, and reasonable assurance engagements, the higher level comparable to a financial statement audit. It can be applied regardless of the reporting framework used, whether that is the European Sustainability Reporting Standards, the ISSB's IFRS S1 and S2, or another suitable set of criteria.
The standard addresses all sustainability topics, from greenhouse gas emissions to workforce and governance matters, and covers information that is often forward-looking or relies on estimates and data from the value chain.
Profession-agnostic by design
A distinctive feature is that ISSA 5000 is profession-agnostic. In many jurisdictions, sustainability assurance can be provided by engineering firms, certification bodies and other specialists, not only by professional accountants. The standard can be used by any assurance practitioner, provided they comply with relevant ethical requirements and quality management standards that are at least as demanding as the IESBA Code and ISQM 1.
This aims to create a level playing field: users of assurance reports should be able to rely on a consistent baseline regardless of who performed the work.
What assurers will focus on
ISSA 5000 places emphasis on the entity's process for identifying the information to be reported — for ESRS reporters, the double materiality assessment. Practitioners must understand how the entity determined which topics are material, even where they do not provide assurance on the assessment itself.
The standard also requires attention to the reporting entity's system of internal control over sustainability information, the use of the work of experts, estimates and forward-looking information, and fraud risk, including the risk of 'greenwashing' through misleading presentation.
Preparing for assurance
Companies can reduce friction by documenting their materiality process, formalising controls over key metrics and establishing clear data lineage from source systems to reported figures. Treating sustainability data with the same discipline as financial data — reconciliations, review evidence, change logs — makes an assurance engagement more efficient.
Boards and audit committees should also clarify who owns sustainability reporting internally and how the assurance provider is selected and overseen.
Conclusion
ISSA 5000 gives the market a common language for sustainability assurance. For reporting entities, the arrival of the effective date is a signal to bring sustainability data up to the standard of control that auditors already expect for financial information.
