This study reviews the sustainability assurance literature to evaluate the intellectual development of the field. It also demonstrates the role of assurance to enhance not only credibility and reputation but also adds more value to firms’ planning, monitoring, and accountability. Assurance level is another notion of sustainability assurance, which is the requirement for sources to be considered as reputable. It highlights the requirement for sources from the aspects of time spent, procedures, and assurance process. This study covers four types of assurance levels, namely reasonable assurance, limited assurance, hybrid, and not specified.