Book excerpt: Nanomaterials have become a key component for energy-related applications. Their design principle, synthesis and applications are well discussed in various scientific and engineering books, but a gap remains in discussions regarding the application of cutting-edge X-ray techniques to these materials. This volume provides insights from the latest development of X-ray techniques to investigate nanomaterials in specific energy fields, bridging the gap between X-ray analytical scientists and material researchers.We aim to provide researchers with a tool to choose suitable X-ray techniques, carry them out with the right procedure, and analyze the data to give the best reliable results. The approach is microscopic and specific. Among the applications emphasized by the chapters in this book are x-ray techniques in heterogeneous catalysis, electrocatalysis for fuel cells, photocatalysis for water splitting and carbon dioxide reduction, organic photovoltaics, and other energy-related applications.