The global energy system is undergoing profound transformations. Tracking the progress of clean energy is essential to deliver sustainable, reliable and affordable energy supplies, as well as to assess overall advancement toward long-term energy goals. This book summarizes the development and deployment status of major clean energy technologies as of 2017 and analyzes forthcoming opportunities and challenges. It also features a dedicated chapter on clean energy innovation, presenting relevant data concerning research, development and demonstration investments made by both the public and private sectors.