The energy sector is undergoing unprecedented change. Twenty years ago, the main concern was having enough oil and gas, whereas today, political leaders are faced with the need to reduce the CO2 emissions produced by still-dominant fossil fuels, without being able to totally rely on renewable energies, which are intermittent and whose share in energy production remains low. Geopolitics and Energy Transition 1 presents the technical aspects of energy and its main characteristics, and outlines the challenges of the energy transition, the conditions for the development of renewable energies and the geopolitical stakes of this transition. It also describes the various energy markets and the consequences of liberalization policies, not forgetting to analyze the structures of the different sectors, while pointing out the fundamental problems of supply security and ways of strengthening it.