First-principles theoretical studies of atomic structures and dynamics on semiconductor surfaces are presented in order to microscopically understand the mechanisms of epitaxial growth and etching of semiconductor surfaces. We first determine the atomic structures of As-rich GaAs(001)-(2 X 4) surfaces and investigate the surface diffusion of Ga adatoms on the As-rich surface, which is one of the most important processes in molecular-beam epitaxial growth of GaAs. Then, to elucidate surfactant-mediated epitaxial growth on Si(001), the site exchange reaction between surfactants and growing atoms is investigated and the reduction of surface diffusion is discussed. Finally, the interaction of chlorines with the reconstructed GaAs(001) surfaces, which plays an essential role in etching, is investigated.