Interface phenomena are most fascinating because of the mixing of different scales and the interference of diverse physical processes. This makes it necessary to use different levels of description: microscopic, kinetic, and gas-dynamical. A unified quasiclassical approach is used to answer practical questions dealing with inelastic gas-surface scattering, the kinetics of adsorption layers, the evolution of inhomogeneities and defects at the surface, the Knudsen layer, the development of boundary conditions on the kinetic and gas-dynamical levels, the determination of exchange and slip coefficients, and so on.