The work presented in this seminar has been developd within the SECURE – Security of Energy Considering its Uncertainty, Risk and Economic implications project, funded by the European Commission, DG Research, FP7, Specific Programme Cooperation (Energy). The project develops tools for evaluating the vulnerability of the EU to the different energy supply risks, and to optimise EU energy insecurity mitigation strategies. In this context, one of the objectives of the SECURE project is the development of adequate energy security indicators for all the major energy sources, in order to identify the risk factors and quantify the EU exposure to volume and price risks in the short and long run. In this perspective, this background report focuses on existent security of supply indicators: on one hand it describes concisely the various methodological approaches currently used to try and capture in quantitative terms the degree of security of energy supply; on the other hand it performs a descriptive analysis of security in the EU of supply based on the historical values taken by some of these indicators. On the methodological side, this report highlights how a common, shared approach is still lacking in the literature about security of supply. On the quantitative side, the application of the available indicators to Europe yields a picture of significant variation among European countries in terms of their dependence form imported fuel and in terms of the vulnerability of their economies, although some clusters of significantly similar countries can be identified. While acknowledging that important information can be derived using available indicators, this review points out to the need of a substantial methodological effort towards harmonizing and fine-tuning the quantitative indicators of security of supply, covering all energy sources, and integrating non quantifiable yet relevant information into the analysis.