The issue of long-term security of energy supply (SES) is a priority topic on the energy policy agenda worldwide. Several factors contribute to raise concerns about the growing energy insecurity, including the following: the current trends in the evolution of global economy and geopolitical changes, particularly the rapid economic growth in emerging economies (e.g. China, India); the strain on oil and natural gas reserves; the concentration of most of these reserves in unstable regions; the threat of sabotage on energy supply infrastructures; the political tension around the Iranian nuclear program, and, finally, the appropriation of the oil and gas sectors in several oil-and natural gas-producing countries by national governments. The issue of energy supply security has been put under the spotlight, especially in those industrialized countries which are very dependent on external sources for their energy procurement. However, the framework in which SES should be assured is controversial. While some decision-makers trust in market instruments for optimizing the energy supply mix, others demandmore government intervention, arguing that the market fails to ensure adequate and sustained levels of energy supply security.