This document is the annual report of the Low Radioactivities Center for the year 1994. The Center is a joint CEA-CNRS laboratory devoted to biogeochemical, climate and Earth science studies. It has developed refined methods for the measurement of small amounts of stable and radioactive isotopes (lead, thorium, cesium, radium, radon, polonium, potassium, argon, beryllium, carbon, oxygen, helium..), in particular isotope dating methods such as K/Ar and C14 methods. The research activities are regrouped in four topics: the study and modelling of great biogeochemical cycles (troposphere physico-chemistry, carbon cycle, mass transfers between atmosphere, ocean and sediments); the evolution of climate (thermohaline circulation and heat transfers); the interactions between the internal activity of the Earth and the Earth`s surface (magnetic field instabilities, oceanic volcanism, geodynamics of orogenic domains, active volcanism); the outstanding events of the Earth`s history (Cretaceous-Tertiary boundary, cosmic phenomena, Quaternary evolution of the fossil man and of its environment). A complete list of the laboratory publications is given in the appendix together with a listing of the other activities (teaching, external collaborations, oceanic campaigns, seminars..). (J.S.). 659 refs., 39 figs., 1 tab., 3 photos., 4 appends.