Historical energy security performance in EU countriesHistorical energy security performance in EU countries Ανασκόπηση Review 2019-10-012018enIt is vitally important for all countries to ensure they have a secure energy supply. This is especially true for European Union (EU) countries, because of geopolitical considerations and ongoing reforms of energy markets. This study applied time-series clustering approaches and three energy security indicators based on the Shannon–Wiener diversity index. The aim was to enhance understanding of how energy security of EU countries, in terms of energy supply, has evolved. An overall improvement in energy security in most EU countries between 1978 and 2014 was identified, with Denmark and the Czech Republic evidencing the greatest improvements. The main driver of improvement has been diversification of primary energy sources. Factors relating to imports (share and diversity of the origins of imports) have also substantially influenced improvement levels. Three groups of countries were identified using cluster analysis: (1) consistently high levels of energy security and showed moderate improvements over time; (2) lower levels of energy security than those in the first group, and also evidenced moderate improvements; and (3) initially low energy security levels, yet followed by significant improvements. The first and third groups were of particular interest because the former reveals best practices while the policies of the latter, in leading to improvement, can serve as guides for other countries.http://creativecommons.org/licenses/by/4.0/Renewable and Sustainable Energy Reviews821737-1748 Matsumoto Ken'ichi Doumpos Michail Δουμπος Μιχαηλ Andriosopoulos Kostas Elsevier Energy security Energy security indicators European Union Hierarchical clustering Primary energy