Modern material technology relies increasingly on processes for surface modification and coating. Generally, we are lacking a possibility to monitor the progress of such processes. Thus the outcome can only be analyzed after the end of the whole process cycle. We are proposing to use spectroscopic ellipsometry (SE) as an on-line monitoring tool. SE, as an optical method, is not affected by high temperatures, process gases, plasmas, etc. It can be used as a monitoring tool or a sensor for closed loop control of processes. The main difficulty is the on-line interpretation of SE data. Depending on the nature of the process monitored or controlled, different models are used for the interpretation. These models predict the SE response depending on different parameters describing the surface under investigation. A fitting process is used to solve the inverse problem, i.e. extracting material data from the SE spectra. We expect increased process stability and shorter development time as a practical benefit from the use of SE.