Physics Colloquium 26th February 2016 delivered by Professor Mark Newton Defects in diamond have great potential for use as quantum sensors and qubits1. Full exploitation of their optical and spin properties necessitates that we control their position, orientation and environment to optimise all of the desirable properties simultaneously. In this talk I will review our understanding of the production, in diamond, of intrinsic defect complexes by irradiation and annealing, and the capture of vacancies and self-interstitials by impurities. New Electron Paramagnetic Resonance (EPR) and optical spectroscopic data will be presented on the production of preferentially orientated defect complexes by electron irradiation and/or annealing whilst the sample is subjected to a large (up to 4 GPa) uniaxial stress. Near 100 % preferential orientation can be achieved for a number of different defects. Furthermore, recent results will be presented where uniaxial stress has been used in-situ to investigate both the reorientation and the spin relaxation properties of the single substitutional nitrogen centre (Ns0) in diamond. It will be shown that uniaxial stress can be used to influence spin diffusion and change spin-spin relaxation.