The aim of the thesis was to examine the effects of the EU's new sustainability reporting directive on the work of an auditor, regarding the required education and assurance process. The legislative reforms related to the CSRD entered into force in Finland at the end of 2023, and with that, the information in the sustainability report must be assured.

The thesis implemented as a qualitative study used a theme interview for data collection among experienced auditors. The results of the thesis clarify that in Finland an authorized auditor only can verify the sustainability report, and in the future, it is still possible to work as an auditor without being also a sustainability auditor. The assurance process of sustainability reporting will be implemented for the first few years at the level of limited assurance, which is based on a review engagement. The quality control of the assurance process and the control of the auditor's independence will be carried out as in auditing.

In summary, the auditing industry had started well in advance to prepare for the changes brought by the sustainability reporting directive. Through their education and work experience, the auditors have knowledge about process controls, including the sustainability reporting audit, and an understanding how the process will be handled in practice. The education, independence, and supervision of the auditor of sustainability reporting occurs along the same protocols similarly as the auditor and by the Finnish Patent and Registration Office.

With the help of the thesis, practices related to the work of sustainability auditors, training requirements and increased general knowledge of the subject have been clarified, which can be used by, for example, students, educational institutions, and employees in the field. In the future, after the first sustainability audits have taken place during 2025, it could be examined how the assurance process in practise has begun, what industry-specific differences can be recognized in the assurance process, and in greater detail how the task will change while stepping from the level of limited assurance to the level of reasonable assurance in the becoming years.