Introduction. The expansion of the use of renewable energy sources (RES) is recognized as one of the means to solve the issue of climate change. At the same time, RES is considered an "antagonist" to traditional natural resources, the latter being unevenly distributed among states, so the formation of an international legal regime for renewable energy requires a balanced approach. In this article, the author analyzes the key component of such a regime, namely, the legal content of the term "renewable energy sources". Based on the documents adopted within international organizations containing definitions of RES, the author establishes the main criteria of RES and formulates a list of energy sources that are universally recognized as such, namely, wind energy, water energy, solar energy, biomass energy, geothermal energy, ocean and river energy.Materials and methods. The methodological basis of the research was formed by general scientific techniques (analysis, synthesis, induction, deduction, comparison, classification, systematization, forecasting) and private scientific techniques (formal legal and comparative legal method).The research is based on the documents of universal international organizations (UN, IEA, IRENA), in which the definition of RES is formulated. The author also resorts to the sources of the EU law governing cooperation in the area of renewable energy as well as documents adopted within the CIS.Research results. The article analyzes the concept of RES in the international law. It is established that it is based on a quantitative criterion (i.e. the speed of reproduction of an energy source should exceed the speed of its consumption) and a qualitative one (its "sustainability" in terms of environmental impact). Since no known energy source is completely environmentally neutral, the "sustainability" criterion is used to identify sources that have significantly less environmental impact than others. The author also identifies a number of energy sources that meet the criterion of renewability, but are not recognized as RES (e.g. nuclear energy), or are not included in the RES, in respect of which states wish to develop international cooperation (e.g. certain types of hydropower, traditional biomass). The author also identifies RES, with regards to which states choose to apply the precautionary principle due to insufficient knowledge of their compliance with the sustainability criterion (e.g. geothermal energy).Discussion and conclusion. The author concludes that the international community has developed an understanding of the criteria for classifying energy sources as RES, and their list is agreed within different interstate communities depending on the actual needs of interaction (for universal organizations, the aim is to mainly the exchange of "best practices" for encouraging RES, for regional ones it also includes developing cooperation mechanisms within the framework of transnational projects and harmonization of state laws). This gives grounds for optimism about reaching a consensus on the concept of renewable energy for the development of an international legal regime.