After a brief description of the employed model hamiltonians, surface phenomena such as reconstruction, relaxation, atomic and molecular adsorption on different surfaces of diamond-like semiconductors have been considered. The interaction of various atoms (H, O, S, F, Cl, Br, I) and some group III metals (Al, Sn) with the (100) and (111) surfaces of carbon, silicon and germanium, have been chosen as examples of atomic adsorption. Simple inorganic and organic molecules (H₂O, H₂S, C₂H₂, C₂H₄, O₂, NO) have been considered to investigate the molecular or dissociative nature of the chemisorption process.