This chapter reviews 40 recent articles from top-ranked accounting journals, addressing the current state of knowledge about quality in sustainability assurance. The articles centre around four themes: understanding the process and quality of sustainability assurance; drivers of assurance quality; the relationship between assurance and the quality of sustainability reports; and the impact of assurance on investor reactions to the reported information. The reviewed literature reflects that the traditional focus of accounting firms may expand to cover not only substantive procedures and error restatements but also judgements on completeness under broadened reporting entity boundaries. We identify the role of specialists and the potential needs for specialisation in establishing sustainability report quality and assurance, especially in the context of detecting ‘greenwashing’. Importantly, we find indications that sustainability assurance does affect investor reactions, but does not necessarily control report quality in the same way as financial assurance. The review acknowledges the view that bringing sustainability assurance under the purview of accounting firms may result in positive spill-over effects on financial audit activities, but may also result in potential negative effects from increased distraction and workload, and decreased independence.