Book excerpt: Energy and its corollary, energy geopolitics, is a more popular issue than ever before in today’s world. After being threatened for 40 years by fears of an oil shortage, we have now entered an era of abundant fossil fuels combined with an increase in global energy demand. However, new fears of sustainable development are now at the heart of energy policy. This book lays the foundation for an understanding of what energy is and the challenges ahead. The book opens with the fundamental principles of energy, reviewing the essential principles of physics that are based on universal laws that never change. It then examines the basics of data analysis and the importance of sustainable development. With this knowledge, it is then possible to review the different energy sources (oil, gas, coal, nuclear, renewable energy, electricity and energy efficiency), explaining how they are produced, the importance of their reserves, their specific markets and the main industrial actors, and the countries that produce them. These notions are essential to understand energy policy and geopolitics. As these are closely linked to its past evolution, many references are provided to historical events that put the current situation in perspective. This educational book is full of graphs, diagrams and boxes to help the reader gradually progress in their understanding of the highly complex geopolitical nature of energy.