Energy security has long been a main driver of energy policies, but its meaning has been contested by policy makers and scholars. The concept incorporates both material and intersubjective aspects, finding different expressions in different contexts and attracting the interest of diverse social actors and academic communities. This chapter identifies, compares, and contrasts five major approaches for analyzing energy security rooted in different scholarly traditions. It argues that in order to facilitate a dialogue among these approaches as well as policy comparison and learning, it is useful to conceptualize energy security as “low vulnerability of vital energy systems.” This definition opens avenues for productive research, unpacking the interplay between material and intersubjective aspects of “vulnerability” and “vitality” of energy systems. Future research should investigate the role of material factors alongside power, values, and trust in defining energy security; explain the gap between energy securitization and action; and explore the interaction between energy security and other energy policy goals.