The book examines the latest trends in clean energy research and applications from around the globe. It introduces various clean energy technologies and discusses efficient energy conversion devices expected to be next-generation power sources. The first section focuses on the effective use of existing renewable energy systems, featuring contributions from internationally recognized experts. It analyzes governance strategies for transitioning to clean energy, including policy instruments and comparative studies of different countries. Key areas highlighted include solar, wind, ocean thermal, biomass, hydropower, and geothermal energy. The second and third sections delve into waste biomass resources and cellulosic materials in biorefineries, emphasizing their efficient use. The final section explores next-generation biofuels, detailing biodiesel production from algae and vegetable oils, and discusses fourth-generation biofuels that use genetically engineered feedstocks. Additionally, it covers clean combustion and emission control in green aviation, focusing on sustainable biofuel Life Cycle Assessments.