Contrary to what is still often admitted when studying the chemical evolution of galaxies, the element abundances observed in stellar atmospheres are often not representative of the initial composition of the protostellar nebula. Element abundances may be altered during stellar evolution by atomic diffusion and/or accretion, if not by nuclear reactions. Atomic diffusion occurs in any type of stars, with time scales extremely different according to the macroscopic motions with which they have to compete. Accretion may occur in solar type stars during planetary formation, which seems to be a common process. Following the success of helioseismology, asteroseismology is now becoming a fundamental tool for penetrating the secrets of the internal structure of stars. In particular it may be used to derive their internal abundances compared to the observed ones. In the present paper, I focus on main-sequence stars. I first recall the situations where abundance stratification can act for exciting or stabilizing stellar oscillations, second I describe how seismic observations can be used to derive abundance gradients inside stars.