Over the last few decades, the phrase “energy security” has spread like an oil spot from specialized literature outward into the standard lexicon of reporters and politicians. Like “security” itself, it is a term whose meaning seems transparent but resists precise definition, in part because the meaning is not immediately obvious and in part because the meaning seems to expand as time goes by. What is “energy security”? Why did it become so prominent in discussions of global politics in the late twentieth century and why is it so important today? We asked Toby Jones, associate professor of Middle East history at Rutgers University and an editor of this magazine, to supply some clarity about this concept. Jones is working on a book that will treat this subject in depth.