During their long existence, metal poor stars have suffered several physical processes which changed the abundances of the elements which are presently observed in their atmospheres. Atomic diffusion leads to the settling of helium and other light elements and may increase the abundances of other ones. On the other hand, the accretion of matter coming from a companion can increase the metallic abundances, but this is limited by the inverse mu-gradient induced instability (thermohaline convection). I propose to give a short review of these processes and their consequences.