This chapter introduces the notion of 'energy security'. The security of energy supply is a central policy concern in many countries. The combination of availability and affordability is at the core of most traditional definitions of energy security, such as the "availability of energy at all times in various forms in sufficient quantities and at affordable prices". Accessibility refers to the geopolitical aspects associated with gaining access to energy resources, while acceptability refers to the social and environmental effects of energy production. Policy interest in energy security has encouraged the development of quantitative measures and indicators through assessments of variation in security can be made. A major implication of developing and using energy security indicators is that a country can actively manage its level of energy (in)security using socio-technical procedures. Energy insecurity is a structural condition, and one that can be made all the more difficult but the efforts of others to enhance their own energy security.