The main purpose of the work reported in this thesis was to investigate and characterise the deep levels in zinc selenide crystals grown from the vapour phase at Durham, and by the CVD technique at AWRE, Aldermaston. The methods of TSC, TSL and DLTS were applied to samples of Durham-grown and CVD ZnSe. The study revealed the presence of traps at 0.14-0.16,0.23, 0.26, 0.30-0.31, 0.45, -0.60 and 0.70 eV in Durham-grown material, whereas the CVD material was found to contain trapping levels at 0.10, 0.20 and 0.27-0.28 eV. The 0.10 eV level in CVD ZnSe is ascribed to the Na acceptor. The traps occurring at 0.26-0.31 eV in both materials are thought to be due to a vacancy-impurity complex. Examination of the CVD material using the EBIC technique in an SEM revealed that the conductivity was limited by electrically-active grain boundaries which were shown to obey a simple potential barrier model. Cathodoluminescence studies showed that the emission bands at 465 nm (edge emission) and 630 nm (copper-red) were quenched at the grain boundaries. A relationship between the absorption coefficient at 10.6 ym and the intensity of the copper-red emission was demonstrated, which has important implications in the production of low absorption zinc selenide for laseroptics.