This research reflects on the legal ascertainment of Customary International Law (CIL). It is argued that CIL is not the result of opinio juris and state practice as the mainstream doctrine generally sustains, but it is rather an Argumentative Framework established, developed and continuously reviewed by the Community of International Lawyers that practice international law. The linchpin of the argument is illustrated in International Environmental Law (IEL), the most dynamic and convoluted field of international law where the functioning of the sources can be more easily observed. To achieve the foregoing purpose, this document initially defends that there is though a Dominant Understanding of CIL, which is usually challenged by alternative theoretical approaches that aim to overcome its inherent flaws. It later demonstrates that there are nevertheless common theoretical groundings among all different approaches that evidence the argumentative construction of this source. It finally develops the main argument conceptualizing both the Argumentative Framework and the Community of International Lawyers that establishes CIL to finally illustrate it in the operation of IEL.