Companies need to consider environmental, economic and social benefits in order to sustain their activities in the long term. Companies that grow up within the scope of corporate governance approach aim to establish balanced relationships with stakeholders and to create value for them, taking into account the public interest.  For this reason, sustainability is one of the important concepts for companies. In this context, it means protecting the resources needed for future generations by ensuring more accurate use of resources today.  For future generations, it is important to create sustainability awareness in terms of the existence of a world that considers the environment and takes into account equality and justice. The necessity of creating this awareness has led the companies to carry out various studies. Sustainability which is a concept that attracts more attention of all social and economic groups, has led to a variety of changes in reporting concepts. The reporting types which were known as “environmental reporting” or “social reporting” so far, now relinquished its place to the type called “sustainability reporting”. Apart from financial data, it is issued that non-financial data must be reported too, as it is issued that there has to be triple reporting in economic, social, and environmental terms. The change was not only in reporting format; there also emerged a new concept named “sustainability expectaion” for stakeholders. Surely this situation has made apparent the requirements in assurance activities about the sustainability expectations and needs that have to be refined in these terms. This has made the need for independent assurance/auditing activities to meet the expectations for sustainability and the need to improve the relevant processes. It is known that; companies aim to protect the interests of stakeholders through corporate governance including fairness, transparency, accountability and responsibility principles.  As assurance services constitute an important mechanism of corporate governance, independent assurance services provided within the scope of sustainability reporting will increase the value of the reports and will also contribute to transparency. The study first explains the concepts of sustainability, sustainability reporting and assurance, emphasizing on the independent/external assurance of these reports.  Then, the study looks into the current situation in Turkey concerning the assurance of these reports, examining the companies included in the BIST Sustainability Index for the period of 2018-2019.