The neutrinos, massive or massless particles?

Neutrino properties, and specially their masses, are a central problem for the grand-unified theories of particle physics and for cosmology (structure formation and dark matter). After an historical introduction, we review th e main experimental efforts on the nu mass determination: direct measuremen ts, search for neutrinoless double beta decays, and finally the search for neutrino flavor oscillations, for which some evidences already exist. (C) 2 000 Academie des sciences/Editions scientifiques et medicales Elsevier SAS.