It is a manual of Public International Law of scientific rigour, whose highly qualified didactic contents involve a rich scientific doctrine on common themes of the normative and material dimensions of the international system, making a detailed study on the full legal subjectivity of the actors who project themselves in the international arena as subjects of the ius contraendi, with respect to the signing of international treaties. Likewise, they intervene as addressees of the opinio iuris or binding element of the customary norms that prove the autonomy of the custom as an inspiring source of the international legal order.The scientific contribution of the author begins in 2009, during the course of his university education at the Chair of Public International Law of the Institute of International Relations of the National University of Kiev "Taras Shevchenco", through the compilation of fragments obtained from the notes and manuals of Ukrainian authors such as: Zadorozny, Bukevich, Cherkes, Rezervska, among others, but does not prevent its linguistic style to be comparable to similar works of the Spanish-speaking world.