The work in this thesis was submitted to The University of Manchester for the degreeof Doctor of Philosophy in March 2016 by Chloe Stott and is entitled "Optically DetectedMagnetic Resonance and Sub-Kelvin EPR at Q Band".In this thesis I will discuss the development, construction and testing of a sub-Kelvin Q- band electron paramagnetic resonance (EPR) spectrometer and optically detected magneticresonance (ODMR) of wide bandgap semiconductors at Q - band.The sub-Kelvin EPR spectrometer was developed to be integrated into a standard commercialsystem. Characterisation of the cryogenics and microwave components of thespectrometer will be discussed as well as the design and adaptations made to enable EPRexperiments to be performed below 1 K. A waveguide thermal break design, previouslyonly used in detectors for the cosmic microwave background radiation, was optimisedusing ANSYS High frequency structure simulator (HFSS) to operate at Q - band and wasbuilt and tested in this spectrometer. The sub-Kelvin EPR spectra of Cr3+ in Al2O3 and[Cr12O9(OH)3(O2CCMe3)15] were obtained. The resonant cavity of the spectrometer wasalso successfully tested at room temperature, with a pulsed microwave bridge, paving theway for further development of the system to enable sub-Kelvin pulsed EPR.A home-built Q - band ODMR spectrometer was used to investigate the wide bandgapsemiconductors ZnO and InGaN/GaN multiple quantum wells (MQWs). The ZnO was anatural crystal, and used to characterise the spectrometer. ODMR from a green c-planeInGaN/GaN MQW was studied to probe the method of carrier localisation in the QW.It was shown qualitatively that the ODMR results support current theories for carrierlocalisation mechanisms, but the noise on these measurements needs to be reduced toprovide quantitative support.