Given the fact that Smart Grid technologies are implemented mainly in distribution networks, it is essential to build a risk-based assessment tool which can model the operational characteristics of distribution networks operation. This thesis presented a distribution network model which captures the features of distribution network restoration, based on approximations of real-time switching actions. It enables the evaluation of complex distribution network reliability with active network control. The development of an explicit switching model which better reflects actual network switching actions allows for deliberate accuracy and efficiency trade-offs. Combined with importance sampling approach, a significant improvement in computational efficiency has been achieved with both simplified and detailed network switching models. The assessment model also provides flexibility for users to analyse system reliability with various levels of complexity and efficiency. With the proposed assessment tool, different network improvement technologies were investigated for their values of substituting traditional network constructions and impacts on network reliability performances. It has been found that a combination of different technologies, according to specific network requirements, provide the best solution to network investments. Models of customer interruption cost were analysed and compared. The study shows that using different cost models will result in large differences in results and lead to different investment decisions. A single value of lost load is not appropriate to achieve an accurate interruption cost quantification. A chronological simulation model was also built for evaluating the implications of High Impact Low Probability events on distribution network planning. This model provides the insights for the cost of such events and helps network planners justify the cost-effectiveness of post-fault corrections and preventive solutions. Finally, the overall security of supply for GB system was assessed to investigate the impacts of a recent demand reduction at grid supply points (for transmission networks) resulting from the fast growing of generation capacity in distribution networks. It has been found that the current security standard may not be able to guarantee an acceptable reliability performance with the increasing penetration of distributed generation, if further balancing service investment is not available.