Energy security has become a popular catch phrase, both in the scientific arena as well as in the political discussion. Yet, in general the applied concepts of energy security are rather vague. This paper sheds some light on concepts and indicators of energy security. First, we conceptually discuss the issue of energy supply security and explain why it is not to handle by one science alone and what economics may contribute in order to structure the topic. After providing a brief survey of existing attempts to define or measure energy security we suggest an additional dimension along which indicators of energy security may be classified: ex-post and ex-ante indicators. Finally, we illustrate our concept on the basis of several simplified indicators. While ex-post indicators are mostly based on price developments, exante indicators are to a greater extent aimed at illustrating potential problems. Our illustration suggests that it is worthwhile to take into account the market structure along with the political stability of exporting countries. (C) 2009 Elsevier Ltd. All rights reserved.