At present, the external assurance of sustainability reporting should be the last link in the corporate reporting process but it is just a voluntary matter from companies. This article presents the issue of external asurance in case of company sustainability reporting. Although, international organizations, such as the IIRC or GRI, have been involved in this process, so far, there is no clear-cut approach to external sustainability reporting. Moreover, there is a question of combined insurance for integrated reports that would help increase the credibility of these types of reports. External insurance remains at the discretion of companies, but in the future it must become mandatory to increase the credibility and quality of corporate reporting.