Particle-grain interactions at implantation energies occur in a large variety of astrophysical sites. The expected physical, chemical and isotopic effects are discussed, as deduced from the analyses of (i) lunar grains implanted with the solar wind ions, and (ii) terrestrial grains implanted in the lab with H, C, N,···, ions. Special emphasis is placed on the amorphisation and the erosion of the grains, and on the synthesis of radicals and molecules within the implanted grains. Implications in planetology and astrophysics are presented.