nickel-nitrogen complexes in diamond. The S2 and S3 luminescence systems are present in natural and synthetic diamond and have been tentatively correlated with the EPR NE3/2 defect systems. Uniaxial stress results indicate that the centres are of low symmetry - typically triclinic. This agrees with the EPR work. Some complementary modelling investigating these systems further supports these results. The thesis is concluded with some discussions and suggestions for further work. This thesis is concerned with the properties of transition metals in High Pressure High Temperature synthetic diamond. They have been studied using the following techniques: optical and electron paramagnetic resonance spectroscopies, and a local density approximation code to model various defect centres. Experimental studies of recently-discovered lines, attributed to Co-N complexes in diamond, are presented. Under uniaxial stress, it is found that a zero-phonon line at 2.367 eV has its optical transition from an E- to A sub 1 -ground state at a defect with trigonal symmetry. Another line at 2.135 eV arises at a defect with monoclinic I symmetry. EPR work has revealed the first Co-related EPR centre - O4. This has monoclinic I symmetry, and, because of hyperfine interaction that results in a broadening of the spectral features, it is concluded that nitrogen is also present. Next, cobalt in diamond has been modelled. Using the ab-initio local density code, AIMPRO, many different cobalt-related structures in diamond have been examined, ranging from the isolated atom to complexes with several nitrogens. The empirical model developed by Ludwig and Woodbury is useful in describing these defects. The defects that have been studied experimentally have also been modelled theoretically. For the 2.367 eV system, a Co-N complex with spin zero has been found to be the most likely candidate for the defect centre. Finally, some optical and modelling work have been combined on