In private law, fiduciary duties are in general linked to fiduciary relationships, i.e., when a fiduciary relationship exists, the parties are deemed to have corresponding fiduciary duties. However, this study shows that fiduciary relationships and fiduciary duties should not always be equated. This article seeks, first of all, to shed light on the interplay between fiduciary duties and fiduciary relationships, as well as to clarify which particular duties can be considered fiduciary. The authors then seek to reveal the specific cases in which fiduciary duties arise and explain their content to ensure the sustainability in these relationships governed by private law. Thereby, a conclusion is made that certain subjects of fiduciary relationships, such as, for example, the manager of the company, the bank and the trustee, have fiduciary duties, but this is not the case for, as an example, the insurer and the doctor.