Cathodoluminescence (CL) of thin films of high Tc superconductors was studied in the scanning electron microscope. The depth and the lateral locations of the different phases can be revealed. In thin films, unlike the bulk superconductors, the CL information can be obtained either from the film itself or the substrate by varying the primary beam energy. At high beam energy, substrate defects and slight thickness variations of a single high Tc phase are observed. The resolution of the CL measurements improves at low temperatures.