Introduction. The 1969 Report of the UN General Secretary noted that 125 fixed platforms were used in offshore areas for the purpose of oil industry. “The International Law of the Sea” (Vol. 1 and 2) published in UK in 1994 provides that “approximately 6000 fixed platforms have been installed in offshore areas throughout the world”. In 2014 only in the area of the OSPAR Convention, 1992, more than 1300 offshore constructions were installed. As for January 2018, the majority of these constructions are in the North Sea (184), the Gulf of Mexico (175), the Persian Gulf (159), and offshore of Far East Asia (152) and Southeast Asia (152). In the other regions this number varies from 3 to 88, while the Canadian Arctic has the smallest number of offshore constructions limited to just 1 object. Accordingly, the issues of interpretation of UNCLOS articles on legal regime of artificial islands and other fixed structures at sea are becoming more pressing. Moreover, the interstate relations, regarding construction of artificial islands in offshore areas, need relevant legal framework to be in place, taking into account the constant growth of economic and military “competition” among countries, as well as the development of science and technologies applicable to offshore construction.Materials and methods. This paper dwells upon the international treaties and other legal instruments, which rely upon the terms “artificial islands”, “installations”, “structures” at sea for regulating relations among States and between States and other international law subjects. The materials for this paper are also commentaries of the UNCLOS, interpretation of the relevant articles of 1958 Geneva maritime conventions, applicable resolutions of the International Maritime Organization, and also definitions of the conventional terms suggested by scholars. The methodological basis of the research consists of general scientific and specific methods, including comparative method.Research results. Most specialists in the law of the sea who research similar topics dwell upon their personal definitions of the conventional terms mentioned above. Such specialists correctly note that the absence of such definitions in UNCLOS makes it difficult to interpret and apply relevant articles in a proper way. Meanwhile, the necessity for such interpretation and application is of practical value because such terms of UNCLOS relate firstly to relevant rights and jurisdiction of coastal and other States in maritime areas where artificial islands, installations and structures are used, including the territorial sea, the EEZ and the continental shelf. Nevertheless lots of different definitions of such terms suggested by scholars make it even more difficult to interpret and apply the rules of UNCLOS on artificial islands, installations and structures, including such sensitive areas of interstate relations as safety at sea and protection of the marine environment.Discussion and conclusions. The authors of this paper do not support any classification or definition of “artificial islands”, “installations” or “structures” or any other definition of other specific words used in UNCLOS for such terms. Instead the authors bring arguments to legal delineation of artificial islands from any other permanent structure at sea independently of the fact which particular word is used in UNCLOS for describing such a structure in a relevant article (“installations”, “structures”, “devices”, “equipment” etc.). It is argued in this paper that such legal delineation is caused by a difficult and comprehensive balance of interests achieved during the III Conference on the Law of the Sea, and such interests relating to economic, military, environmental interests of States, coastal and non-coastal, firstly in the EEZ and on the continental shelf.