[en] Within the context of construction of the Jules Horowitz reactor in Cadarache, this publication briefly recall some generalities about research nuclear reactors, and indicates their classification depending on their purpose (fundamental research, material and nuclear fuel tests, safety tests, core physics measurements, teaching and training). It comments the design approach of research reactors (design principles, differences with a power reactor). It presents the different types of research reactors: heavy water moderated, light water moderated (pool reactors with open core, pool reactors with vessel), and other types (TRIGA reactors, fast breeder reactors). It proposes an overview of the main usages of research reactors: fundamental research, applied and technological research, safety tests, and other usages (production of radio-isotope for medicine and for industry, analysis by neutron activation, non destructive examination by neutron radiography, doping of silicon monocrystalline ingots). It comments the relationship between research reactors and issues of non proliferation, but also the issue of safety of research reactors. It indicates and briefly presents research reactors operated in France and their usage