Although it is generally acknowledged that the concept of due diligence originated in the areas of the law of neutrality and the regulation of the treatment of aliens, it has gradually spread to many other areas of international law, including international environmental law or investment law. It is clear that specific aspects of due diligence in individual sectors of international law deserve special attention. However, this paper focused more on the origins and development of the concept in terms of rules on the responsibility of States for internationally wrongful acts. However, despite its growing occurrence and popularity in treaty practice and case law, it remains a relatively difficult concept to classify at the theoretical level. This is because due diligence is not by its nature an alone standing rule of international law which would impose an immediate duty of conduct on States. Instead, this concept is included in primary rules, which require due care from the State, usually as part of an obligation to prevent a certain damage. It follows that the content of responsibility depends both on the specific obligation (in the given primary rule) and on the assessment in concreto of all circumstances of the case, whether the given conduct of the State (means or efforts adopted) was reasonable and proportionate in these circumstances. It is a general concept with flexible content that allows for the determination or exclusion of State responsibility.