Electricity systems represent a major source of global pollutants. Whilst currently relying heavily on fossil fuels, electricity systems are progressively shifting towards renewable sources to mitigate climate change and enhance energy security. The goal of this PhD project was to develop a systematic framework for the life cycle assessment (LCA) of electricity systems, which aimed at providing: •Scientifically sound recommendations for decision-making processes, leading to more sustainable energy systems; •Accurate and transparent LCA data for electricity supply, thereby increasing the robustness of LCA results for a multitude of products producing or consuming electricity throughout the lifecycle. The main findings in relation to: (i) electricity generation, (ii) power transmission and distribution and (iii) low-carbon electricity systems are reported in the following paragraphs. A great deal of variability was found in the literature regarding LCA of electricity generation in terms of modelling methodology and power plant characteristics, both of which strongly affected the results of the LCA. Major issues for individual electricity generation technologies were identified and discussed. For example, electricity used during the manufacturing of the power plant, reference year and data collection approach (process-chain or input-output analysis) strongly affected the impacts of hydro, wind and solar power. This information needs to be documented, to ensure comparability between studies. Based on information gathered from the literature, typical emission factor ranges for each technology were provided. Results showed that emission factors per unit of energy input should be used for thermal conversion processes (as opposed to emission factors per unit of electricity produced), as the efficiency may vary depending on the operation of the plant within the power system. The choice of LCA approach used to solve multi-functionality for combined heat and power plants strongly influenced how the environmental impact of electricity produced at such plants was estimated. When it is not possible to expand the assessment’s system boundaries, exergy allocation should be used, as it is more consistent with the general principles of LCA. Lastly, land use changes (LUC) were found to increase greenhouse gas (GHG) emissions from energy crops to levels comparable to those of fossil fuels; consequently, it might be preferable to use energy crops for purposes other than producing electricity. Transmission and distribution of electricity are often not included in LCA of power systems. An LCA of the Danish transmission and distribution systems was performed, showing that the distribution network makes a significant contribution to the impacts of electricity delivered to customers. In the future, because of the implementation of smart grids and low-carbon electricity systems, these results might change radically. It is thus recommended to include transmission and distribution in future LCA studies, while developing data on smart grids should be a priority for future research. The environmental impacts of low-carbon electricity systems were assessed by combining LCA with power system modelling. Possible scenarios for the island of Ireland in 2025 and Denmark in 2030, with high amounts of wind power, were developed using Unit Commitment and Economic Dispatch, including wind and demand forecasts. This approach allows for assessing the influence of the fluctuating nature of wind on other electricity sources – this was not found in the LCA literature on renewable-based electricity systems, as it is based mostly on aggregated modelling. The results showed that an increase in wind power causes greater emissions from other power plants in the electricity system (which need to ‘cycle’ – adjust their production – more frequently); however, considering the entire electricity system, increasing wind power penetration reduces the overall emissions. Electricity storage limits the amount of cycling but environmental benefits are related to the base load fleet in the system, i.e. having coal as base load causes an increase in emissions. Electricity imports and exports are likely to increase with the expansion of wind power: transparent LCA modelling and adequate data for neighboring countries’ power systems are hence important for reliable and usable results. Focusing on the Danish electricity system, it was found that using energy crops for electricity production did not lead to GHG reductions, owing to LUC-related impacts. Conversely, it will be possible to reduce GHG significantly, by increasing power production from residual biomass and wind and decreasing electricity production based on fossil fuels.