Energy security studies have expanded from their classic beginnings following the 1970s oil crises to encompass various energy sectors and increasingly diverse issues. This viewpoint contributes to the re-examination of the meaning of energy security that has accompanied this expansion. Our starting point is that energy security is an instance of security in general and thus any concept of it should address three questions: “Security for whom?”, “Security for which values?” and “Security from what threats?” We examine an influential approach – the ‘four As of energy security’ (availability, accessibility, affordability, and acceptability) and related literature of energy security – to show it does not address these questions. We subsequently summarize recent insights which propose a different concept of energy security as ‘low vulnerability of vital energy systems’. This approach opens the road for detailed exploration of vulnerabilities as a combination of exposure to risks and resilience and of the links between vital energy systems and critical social functions. The examination of energy security framed by this concept involves several scientific disciplines and provides a useful platform for scholarly analysis and policy learning.