Background: There is an increased demand for professions in sustainability assurance due to the forthcoming directives of CSRD coming in 2024. Currently, there is a lack of knowledge of what a sustainability auditor does to build audit comfort and ensure audit quality in a sustainability report. Therefore, it is important to devote time and attention to this developing industry as studies have found issues in reporting on sustainability.    Purpose: The purpose of the thesis is to get a better understanding of how sustainability auditors may build their comfort in sustainability auditing despite its complexity, and variability and that they are challenging to fully comprehend due to a lack in standardization. This study aims to extend existing research by contributing with a deeper theoretical and empirical knowledge of sustainability auditing, based on structure as well as professional judgment.    Methodology: An explanatory purpose and an abductive approach have been applied to the method. The study is based on a qualitative research method as nine interviews have been conducted in a narratively and semi-structured way. These interviews, together with one participant observation have formed the empirics.    Conclusion: The thesis builds an understanding of the comfort theory in combination with sustainability assurance in the comparatively new area of practice within auditing. The findings show that the sustainability auditors find comfort in a combined use of professional judgment and structure, but today the professional judgment is mainly used to conduct the audit comfort because of the lack of structure.