The dating of hominids prior to Homo Erectus was based until now on the isotopic geochronology of rocks and minerals of volcanic origin in stratigraphic relationship with fossils. Two methods have been widely used: potassium argon and fission track dating. The dating of volcanic materials -- lava flows, tephras -- combined with remains of hominids presents numerous difficulties essentially linked to various types of contamination and alteration. Two examples provided by the eastern African Rift illustrate the problems linked to such datings. Recent improvements in the dating methods could contribute on the one hand to a better use of potassium argon methods and of fission tracks in volcanic materials. And on the other hand, with the introduction of new methods such as thermoluminescence and electron spin resonance dating, it would seem possible to date directly either the fossil bones, or the combined sedimentary materials. This opens up new prospects, especially for all the sites which are not interstratified with volcanic materials. -- AATA