In situ spectroscopic ellipsometry was applied for real-time control of the reactive magnetron sputter process of optical coatings on glass. Single transparent films of SiO2, Si3N4 and SiOxNy with varying composition as well as multilayer optical filters on transparent thick floatglass were fabricated. The process control system is based on an optical monitor for the deposition rate and the film composition in combination with a plasma control setup. The latter was developed for the short-term stabilization of the mid-frequency magnetron sputter process. The quality of the filters deposited using the control system is demonstrated by comparing the calculated reflectivity and transmission with the experimental ex situ data. The investigations show that for silicon oxynitride films, the combination of plasma control with ellipsometric control can be used for controlling the stoichiometry of the growing film.