General principles of law are basic rules whose content is very general and abstract, sometimes reducible to a maxim or a simple concept. Unlike other types of rules such as enacted law or agreements, general principles of law have not been “posited” according to the formal sources of law. Yet, general principles of law are considered to be part of positive law, even if they are only used as subsidiary tools. They constitute necessary rules for the very functioning of the system and, as such, are inducted from the legal reasoning of those entitled to take legal decisions in the process of applying the law, notably the judiciary. They also constitute integrative tools of the system as they fill actual or potential legal gaps. In international law, general principles of law have been the object of much doctrinal debate based on the different meanings attributed to the notion and the theoretical problems that they pose. Much confusion derives from the use of the expression “fundamental principles of international law” that is at the top of the legal system and originates in treaty or custom (e.g., the principle of sovereign equality of states or the principle of the prohibition of the threat or the use of force) and that will not be dealt with here. Given the wording established in Article 38, paragraph 1(c) of the Statute of the International Court of Justice (“general principles of law as recognized by civilized nations”), the question of the origin of general principles of law as applied at the international level has also been a matter of controversy. The common perception is that these principles find their origin in the domestic legal systems. Once there is the conviction that some of these general tools are commonly shared principles that can be found in the domestic systems, they can also be applied in international law. They are logic inferences that can be found in any legal system: the principle of reparation for caused damage, the principles of interpretation of rules, or those used for the resolution of conflicts of rules—many of them known through Latin maxims—are good examples. The judiciary has also developed a number of general principles of law, such as audiatur et altera pars, actori incumbit onus probandi, or the fact that the judge of merits is also judge of the incidental jurisdiction. However, they are also logic inferences that are related to particular areas of international law, giving room for the emergence of general principles specifically applicable in the realm of international law, for example the principle of humanity in international humanitarian law. Given the definitional problems that the notion poses, the first references will be to works and jurisprudence on the topic so as to set the field under scrutiny. Likewise, the various histories that have been recounted about the notion’s emergence will be dealt with before engaging with definitions and other more substantial matters. The authors wish to thank Ms. Lena Holzer (PhD candidate, Graduate Institute of International and Development Studies) for her helpful research assistance.