Cathodoluminescence is a tool that is used to investigate optical emission from semiconductors. The technique operates by exciting the sample with a focused electron beam such that it becomes excited and emits photons which are then analysed. The technique can be operated with a continuous electron beam but can also be utilised in time-resolved mode by pulsing the electron beam. In this thesis, steady state and time-resolved cathodoluminescence are used to investigate the properties of III-nitride semiconductors. The instigation is split into three main research topics which will now be summarised. Firstly, cathodoluminescence will be applied to the investigation of InGaN/GaN core-shell nanorods for the purposes of light emission as light-emitting diodes. This work focuses on understanding the structural and optical emission inhomogeneities across the nanorods and their relationship to their fabrication processes. Importantly, the nanorods have exposed non-polar, semi-polar and polar surfaces. By using time-resolved cathodoluminescence, the difference in charge carrier dynamics at these exposed crystallographic facets will be investigated. This is carried out to assess the effect of polarity on the degree of carrier recombination. Secondly, cathodoluminescence will be used as a novel quantification technique in buffer structures of high-electron-mobility transistors. The idea here is to quantify the change in the alloy composition from the AlGaN layer of the structure and the carbon doping concentration from the GaN layer. The change in alloy composition is quantified by considering the change in emission wavelength while the carbon concentration is evaluated by utilising the change in emission intensity. Lastly, cathodoluminescence is applied to assess the radiation damage in GaN layers from Ga and Xe ion milling processes. Here, the emission intensity is used to compare the level of damage produced by the two milling species.