A rule (norm) of international law must be derived from one of its recognized sources. Given the absence of a central legislative organ in international law, States have determined in advance those sources from which a binding international norm may emerge.

The formal sources of international law are enumerated in Art 38 International Court of Justice (ICJ) Statute. They include treaties, international custom, and general principles of law. In addition, Art 38 enumerates two subsidiary sources, namely judicial decisions and scholarly writings. There is no hierarchy among the formal sources, especially between custom and treaty; it is accepted that they may coexist alongside each other.

Besides these formal sources, theory also recognizes so-called soft law. This is not binding as such but is considered as having persuasive value which may through consistent practice and usage transform into customary law. Examples of soft law include key resolutions of the UN General Assembly.