De-centralised wind energy is proposed as a future renewable electricity generation technology. The opportunity for individuals or small organisations to generate power locally enables a reduction in losses associated with long distance electricity transmission. As the majority of Europe's population live in close proximity to one-another, 'point of use' wind energy systems will predominantly operate from within the urban environment. Urban winds are characterised by increased levels of 'gustiness' and a decreased energy content with respect to rural and offshore sites. Vertical axis wind turbine designs, capable of handling winds comprising frequent changes in direction , are proposed as the configuration of choice for the urban environment. The evaluation and optimisation of urban, vertical axis wind turbines requires a broad systems-based approach, consisting of: an assessment and simulation of the urban wind resource, the development of unsteady aero-mechanical turbine performance prediction models and the inclusion of typical installation losses. A wind turbine 's location is found to be far more important in terms of net energy extraction than a particular turbine 's aerodynamic efficiency. To quantify the influence of location on overall energy yield, an in-depth evaluation of the urban wind resource is completed , including the development of methods to predict terrain roughness heights (shown to be a key factor dictating a site's total available wind energy and levels of gustiness) from single point wind speed data. When compared to offshore locations, urban winds are characterised by: a reduction in mean wind speeds, an increased 'gustiness' (from negligible values to 23% of the total available energy resource) and, depending on whether kinetic energy contained within an unsteady urban wind can be extracted, a fall of between 65% and 75% of the total available wind energy resource. This large decrease in available energy is viewed to be extremely significant, indicating that, for an urban turbine to be competitive with its offshore counterpart (at the same elevation) an increase in overall efficiency of the decentralised urban wind turbine system (compared with the offshore system) of between 286% and 400% is necessary. Unfortunately short period gustiness is shown to be detrimental to turbine performance, hence a wind turbine operating within a steady wind will always extract a larger proportion of the total available wind energy than a turbine operating within an unsteady wind. A time and space accurate free-vortex model capable of simulating turbine performance in a fluctuating wind as well as accounting for turbine-turbine interactions has been developed. Combining this vortex model with formulations for inertial effects, active control models and installation losses, has enabled a systems based design optimisation of wind turbines operating within urban environments. Rotor designs with very benign (flat topped) power curve characteristics, at the expense of maximum aerodynamic efficiency, are shown to limit the adverse effects of an urban wind's short period gustiness on net energy output. For the small urban rotors considered (15m2), energy losses incurred converting turbine shaft power to grid conditioned outputs (240V, 50Hz) are seen to drive a systems design and subsequent optimisation, with these installation losses sometimes exceeding total aerodynamic generation, leading to negative net energy outputs (a net flow of energy from the national grid to the wind turbine system). Optimisation of both aerodynamic rotor design and turbine control strategies illustrate that improvements from a baseline configuration (with an initial positive net energy yield) of up to 267% are possible . Incorporating these improvements within a typical 15m2 urban wind turbine installation result in predictions of annual energy yields of up to 3200 KWh, amounting to ~ 13% of the total annual energy use of an average adult living in the UK. The installation of turbines on the tops of tall buildings is suggested as a method to mitigate the very low energy yields observed for urban turbines. The space available for turbine installations on the tops of these tall buildings is limited and close spaced turbine operation is likely to result. A study of turbine-turbine interactions shows that a blockage effect, similar to that observed in wind tunnel tests, results in favourable turbine performance. Thus, in a wind with a strong prevailing direction, a close spaced turbine array is capable of a better steady-wind aerodynamic performance than the same number of isolated wind turbines.