This chapter provides an overview of the genesis and transformation of the notion of energy security over time, outlines the prevailing shifts in patterns and concepts of energy security in the twenty-first century, and discusses attempts to conceptualise energy security. It also offers a novel conceptualisation of energy security and compares available approaches with the proposed concept. The chapter defines energy security as sustainable use of renewable energy sources (RES), which means the purposeful and ever-increasing utilisation of renewable sources for the production of power, and a corresponding decrease in reliance on non-renewable sources, complemented by resource efficiency programmes. The chapter emphasises the need for effect...