This thesis assesses the future security of the UK electricity system in a low-carbon context. Electricity provision is a crucial and ubiquitous component of industrialised societies, and over the past couple of decades a number of fundamental changes to electricity systems have meant that the security of this provision has taken a central place on the policy agendas of the UK and many other industrialised nations. Alongside this, emerging normative, legal and political imperatives to mitigate climate change mean that energy systems will need to undergo a fundamental transition. The overarching aim of this thesis is to assess the future security of the UK electricity system in a low-carbon context, in order to identify the main risks, trade-offs and synergies which may emerge between different objectives in a transition to a low-carbon electricity system. To do this, this thesis develops a set of indicators for assessing the electricity security of low-carbon transition pathways, building on assessment frameworks from the existing literature and utilising a range of both quantitative and qualitative indicators. The indicator set is used to assess the security of three pathways for the UK electricity system, each of which aims to meet the UK’s 2050 greenhouse gas reduction target. The indicators are then used as the basis for interview discussions with 25 experts from the UK energy sector, in order to explore the diversity of perspectives in the UK energy community. Finally, the experts’ perspectives are used as multiple ‘lenses’ through which to view the results of the security assessment of the three pathways. This thesis makes a contribution to knowledge and understanding in three ways. Firstly, it makes a methodological contribution by proposing and testing a set of indicators to measure the security of electricity systems in long-term scenarios of national energy transitions. The thesis takes an interdisciplinary approach and utilises both quantitative and qualitative indicators without aggregation in order to identify synergies and trade-offs. Secondly, this thesis makes an empirical contribution by applying this set of indicators in a novel way to assess the security of a set of low-carbon transition pathways for the UK electricity system: this is the first time that such a comprehensive security dashboard has been used to assess a set of future electricity system scenarios. By including reliability and cost parameters alongside a range of other important aspects of energy security such as diversity, trade and acceptability, this thesis extends the empirical work of existing frameworks to explore the potential implications of a low-carbon transition on electricity system reliability and costs, and the potential trade-offs between various objectives. Thirdly, this thesis makes a further empirical contribution by identifying the diversity of perspectives amongst UK energy experts; this is a novel contribution to the energy security literature, which contains few empirical studies on experts’ perspectives on energy security, and no previously-existing work of this kind in the UK context. Finally, this thesis analyses the impacts of these perspectives on the results of the security assessment, thus providing the first study of this kind to actively incorporate multiple perspectives on energy security into an indicator assessment. The thesis finds that the three low-carbon pathways tested against the indicator framework all demonstrate a reduction in flexible, responsive supply capacity compared to the 2010 baseline, which could reduce the ability of the system to respond to unexpected perturbations in the supply/demand balance. The results show that demand reduction may be highly beneficial and results in co-benefits across multiple security dimensions (although this thesis has not conducted detailed investigation of the costs and risks of demand reduction, and therefore this issue needs to be analysed further in future research). Increasing the penetration of renewable electricity generation is shown to increase the diversity of the generation mix, and to have a positive impact on greenhouse gas emission reduction and resource depletion; however, it could lead to a reduction in system balancing capability, and does not necessarily minimise dependence on fuel imports. The decentralised transition pathway is shown to have the fewest ‘red flags’ of security risk in the longer-term; this finding is an interesting addition to the academic and policy literature which has debated the potential security benefits of a decentralised electricity system for the UK. However, this thesis also highlights that there are many areas of uncertainty and potential security risk in a transition to a decentralised electricity system, which may experience some aspects of heightened security risk in the medium-term.