It achieved this result by diversifying the energy mix, and in particular the electricity mix, as well as by reducing the overall share of fossil fuels imported from outside the OECD area. Consistent with the mandate of the Ad hoc Expert Group on Nuclear Energy and Security of Supply “to identify a relevant quantitative approach to measuring the contribution of nuclear energy to security of supply”, the study then presents a broad range of indicators and models that quantitatively assess a country’s level of security of energy supply. It subsequently develops a specific composite indicator that allows the measurement of the level of security of energy supply as well as the contribution of nuclear energy over the past 40 years, for those OECD countries for which a consistent data set was available.