This work presents the results of an investigation into the technique of combined TEM-Cathodoluminescence and its application to the study of GaN epitaxial layers grown by MOVPE and PAMBE on sapphire and LiAlO(_2) substrates respectively - and MOVPE grown In(_x)Ga(_1-x)N/GaN/A1(_2)O(_3) QW structures. The measurement of CL in a TEM allows spectral information to be correlated with structural information. In-situ electron beam degradation curves of panchromatic CL from GaN epilayers and In(_0.1)Ga(_0.9)N QW emission revealed a decline in the luminescence which could be attributed to the introduction of non-radiative recombination centres. The influence of thickness on both CL spectra and images was investigated experimentally and by modelling. A method of normalising STEM-CL images for thickness contrast was developed. Application of this normalisation to In(_0.1)Ga(_0.9)N QWs in cross-section revealed inhomogeneous CL with bright regions 200-700nm in width. No systematic relationship was identified between luminescence at the QW peak emission wavelength, QW(_A), and luminescence at QW(_A) ±10nm. This finding does not support the hypothesis that variation in QW CL brightness is due to local compositional fluctuation. However, clusters of threading dislocations were shown to suppress QW luminescence and are suggested as a cause for the observed inhomogeneity in luminescence. A statistical analysis of (dislocation related) V-pits in In(_x)Ga(_1-x)N MQW samples revealed clustering of pits on a length scale of 60-120nm, but no long range clustering indicative of sub-grain boundaries was found. Finally TEM-CL spectra and monochromatic line-scans were used to show that bundles of basal plane stacking faults in M-plane GaN epitaxial layers grown on LiAlO(_2) emit radiatively at 3.3-3.35eV (l00K). The radiative transition energy is consistent with models in the literature that consider basal plane stacking faults to be layers of cubic GaN in the wurtzite matrix which act as type II QWs.