The electron beam used to generate SEM images often results in the emission of light (cathodoluminescence). If the luminescence is detected, a new and distinctive image of the sample is produced. This new CL-SEM image distinguishes regions of high and low luminescence efficiency. Cathodoluminescence spectroscopy is an irreplaceable research tool as it allows the researcher to obtain a correlation between structural and optical properties and well as providing high spatial resolution of the luminescence distribution in the sample. In this study we apply CL-SEM to the study of GaN. Normal hétéroépitaxial growth of GaN on substrates such as AI2O3 leads to a columnar material consisting of many hexagonal grains. The tilt and rotation of the grains result in a high dislocation density of the order of 1010 cm'2. The effect of this high dislocation density on the luminescence efficiency was studied. In the Epitaxial Lateral Overgrowth (ELO) process, the material grows vertically through the etched windows in the SiOa mask and then laterally over the mask. This leads to a reduction in the dislocation density by three to four orders of magnitude. The results reported here confirm that the luminescence efficiency is substantially higher for ELO material. We report also on photoluminescence measurements of a blue defect band in GaN which has received considerably less attention than the characteristic yellow band. The blue band appears for samples cooled in the dark, and it disappears under illumination at a rate that depends on temperature. We analyse the decrease in the blue luminescence and a corresponding increase of the yellow band as a function of time at a range of temperatures and suggest possible explanations for the complex nature of the metastability displayed by the defect responsible for the blue band.