Sustainability reporting aims to communicate an organization’s sustainability performance to stakeholders. Integrated reporting is designed to provide information on how well the organization has managed its financial and non-financial capital to the providers of that capital. However, the credibility of these documents is often questioned by stakeholders; i.e., are the claims made within these reports reliable, and do they provide a balanced account of the reporter’s performance? Responding to stakeholder scepticism, organizations have begun to secure external assurance over their sustainability reports and integrated reports. This chapter explores these new forms of assurance, looking first at the demand and supply side of the sustainability assurance market and subsequently exploring issues relating to the assurance of integrated reports.