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The spins of phosphorus doped in silicon are potential candidates for a quantum computing device, with models based on the use of nuclear and/or electron spins suggested. For a quantum computing device, several essential criteria must be demonstrated before any physical implementation, and these include qubit control gates, long decoherence time and scalability. Scalability and compatibility with existing fabrication technologies are strong points in favour of a silicon based system. For spin based schemes, silicon has the potential to provide a host with zero nuclear spin (isotopically purifed 28Si) and also the phosphorus donor provides both nuclear and electron half integer spins (ideal case). In this work, a magnetic resonance method (electron spin resonance) was utilised to investigate these critical issues (controllable quantum gates and decoherence time) for the electron spins of phosphorus donors in silicon. Electron spin resonance (ESR) studies of an ensemble of phosphorus electron spins in silicon were conducted via both continuous wave and pulsed methods. For pulsed ESR operations, two low temperature (4 K and millikelvin) X-band pulsed ESR systems were built. They were designed especially to suit Si:P decoherence time measurements. The design, modelling, construction and evaluation of the probe heads are described. With the aid of computer simulations, the performance of the probe heads was optimised and a rectangular loop gap resonator was found to be the most suitable for wafer type samples. The resonant frequency, quality factor, and coupling coeffcient were calculated via simulation and are in reasonable agreement with experimental results. This demonstrates the effectiveness of such simulations as a tool for optimising the probe head performance. A millikelvin pulsed ESR system was set up through the combination of a dilution refrigerator, superconducting magnet and the in-house construction of a pulsed ESR spectrometer. This novel system allows pulsed ESR experiments on an ensemble system to be realised down to the millikelvin temperature range, hence providing conditions considered most favourable for quantum computing studies. The use of light in combination with the pulsed ESR systems was also explored in an endeavour to overcome the problem of very long spin-lattice relaxation time, T1, allowing the decoherence time to be measured more effciently. With these novel low temperature pulsed ESR units, two-pulse electron spin echo experiments were conducted on phosphorus donors in silicon (both natural silicon (natSi) and 28Si) with the phosphorus concentration in the range of 1015- 1016 P/cm3 and to lower temperatures than previously investigated. Decoherence times measured for both natSi:P and 28Si:P (with similar donor concentrations) were longer than previously reported. Discussions on several effective ways to obtain even longer Si:P decoherence times including variations to sample configurations and experimental conditions are presented. In addition to the pulsed ESR studies, the Si:P controllable quantum gate functions, A gate and J gate, were examined by the continuous wave technique via Stark shift and exchange interaction experiments respectively. Stark shift experiments on bulk samples were carried out to investigate possible manipulation of the spins by the applied electric field. Continuous wave ESR was also used to examine low energy ion implanted Si:P devices, both by single (P+) and dimer (P+2 ) implanted donors. The outcomes from these studies provide materials information useful in formulating a strategy toward the Si:P device fabrication via the top down approach.