The present article covers both the theoretical aspects of surfactant adsorption and the modern experimental methods for studying the properties of adsorbed surfactant layers. The principles of ellipsometry, photon correlation spectroscopy, small angle neutron scattering, and neutron reflectivity are explained in detail. A special attention is paid to the surface tension relaxation in micellar solutions, the properties of mixed surfactants, the wetting enhancement, and the damping effect of surfactant films on capillary waves