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 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.