This chapter introduces the meaning of energy security. It starts by presenting different definitions of energy security. It then discusses different perspectives of energy security by looking from the point of view of energy importers, exporters, transit countries, and military. By the end of the chapter, the main energy security challenges, risks, and vulnerabilities are discussed in order to problematize energy security and better grasp the interconnectedness of the concept. The chapter ends with a discussion of the effects of climate change and renewables for energy security.