An apparatus for monitoring a temperature and/or a temperature-dependent parameter of an optical element arranged in a path of a laser beam includes a measuring light source to produce measuring radiation and direct the measuring radiation to the optical element, a detector arranged to detect a portion of the measuring radiation that has passed through a passage region of the optical element, an evaluating device connected to the detector to monitor the temperature and/or the temperature-dependent parameter. The evaluating device is configured to determine the temperature and/or the temperature-dependent parameter based on an intensity of the measuring radiation detected by the detector in the passage region.