This paper presents an optical method of process control based on the thermographic detection of signals from the process zone. At the Laser Zentrum Hannover e.V., a monitoring system based on a high-temperature camera has been developed to enable on-line monitoring of the cut quality. During the cutting process, the temperature field was observed on-line using a thermocamera. The camera images were analyzed afterwards using specific digital image analysis. The crucial task of the work is the adequate application of analyzing and visualization techniques to extract significant information from the measured signals, and to find correlations to the cutting quality. Investigations have been carried out on cutting metal sheet materials such as steels and titanium. For the investigations, different process parameters such as laser power or process gas pressure were varied. Influences of the optical set-up on the signal were studied. Results show that the temperature distribution in the process zone is strongly connected to the cut quality, e.g. dross attachment, surface roughness of the cut kerf, width of the kerf. Observation of the temperature and temperature gradients at the cutting front allow a determination of the resulting cut quality.