After preceding discussion it is obvious that this can again be only a partial input to the ongoing discussion about what energy supply security means for different countries and different stakeholders. This index is normalised to range from 0 (extremely low security) to 100 (extremely high security). It covers final energy demand, energy conversion and transport and primary energy supply and, hence, in principle the entire energy system.1 The S/D Index is also at the basis of the SSDI employed in Chapter 3 of this publication to track the evolution of the energy supply situation in selected OECD countries over the past 40 years in a systematic manner. The difference in the SSDI and the S/D Index is essentially that the SSDI was adapted to be able to work with the only available consistent data available for the past 40 years, i.e. the IEA Energy Statistics.