An outline of optical spectroscopies of semiconductor surfaces based on reflectance of ultraviolet and visible light is presented. The principle of the two main differential techniques which are widely used are discussed: the surface differential reflectance spectroscopy (SDRS) gives the change of reflectance of a clean surface due to chemisorption of foreign atoms or molecules; the reflectance anisotropy spectroscopy measures the difference along the two principal directions in reflectances of an anisotropic surface, either clean or adsorbate-covered. The ability of these techniques for investigating surfaces is illustrated by examples on different semiconductors (Si, Ge, III-V). In particular, it is shown that the comparison of experiments with theoretical studies allows us to get deep information on the structure of the surfaces, their reconstruction, and their changes induced by temperature or by adsorption. Monitoring the adsorption of gas, including the determination of adsorption sites and the kinetics of chemisorption, is also addressed.