Energy security is a multidisciplinary field which overlaps with engineering and energy systems analysis, earth sciences, economics, technology studies, political science, international relations, and security and military studies. Though discussions of energy security have been around for most of the 20th century, a systematic “energy security science” has emerged only recently and is still a young and dynamic field. The structure and the borders of the area are widely debated and contested. Contemporary debates on energy security include dilemmas such as whether energy security is a “socially constructed concept” or an objective property of energy systems, whether it is primarily a national-level issue or whether it also exists at different scales (household, local, regional, and global), whether it relates only to conventional or also to “human security,” whether it is a generic or context-dependent idea, and how it relates to other energy policy issues (e.g., environmental and social impacts). The key outcomes of this debate include the idea that energy security relates to both shocks and stresses, includes both physical and economic aspects, and relates to the “vital energy systems” which underpin the stability and functioning of societies.