Sustainability reporting is a process that assists organizations in understanding the links between sustainability related issues and the organization’s plans and strategy, goal setting, performance measurement and managing change towards a sustainable global economy – one that combines profitability with social responsibility and environmental care. The final product of this process is a sustainability report where the organization reports on the most critical (or material) aspects of the organization’s economic, social and environmental impacts and the relation of those with its performance. This publication aims to help sustainability reporters and report readers understand the external assurance of sustainability reporting. In doing so, GRI hopes to clarify and further improve the use of external assurance.