Photoluminescence measurements of three cadmium related defects are presented in this thesis Group I consists of five zero-phonon lines with the minimum energy line observed at -1058 1 meV. A second group of lines, labelled group II is found to be broadly similar to group I and is centred at ~ 1068 meV Group III consists of three zero-phonon lines, labelled a-i, a2 and 0C3 with ai observed at -1083 3 meV. Temperature dependence, uniaxial stress, isotope substitution studies and Zeeman measurements were performed on the three defects reported. In the case of groups I and II, temperature measurements have shown excited state manifolds with thermalisation occurring to a single ground state. For group III thermalisation data show that both the ground and excited states are manifolds of three levels each, with strong selection rules governing the transitions Uniaxial stress measurements reveal trigonal (C3V) symmetry for groups I and II Group III is assigned to monoclimc I symmetry. Cadmium isotope substitution experiments reveal that all three centres contain at least one cadmium atom. The diffusion of lithium into cadmiumdoped silicon enhances the luminescence of all three defects. For group II, lithium isotope substitution studies indicate the involvement of two Li atoms in the defect. Zeeman measurements show the excited state manifolds consist of both singlet and triplet states for the three centres, with isotropic triplet state g values close to 2.