Renewable energy is the energy generated from renewable natural resources, and it widely includes solar energy, geothermal energy, biomass energy, wind energy, ocean energy, etc. Renewable energy not only contributes to the increasing energy need but also benefits the worsening environmental situation because renewable energy is renewable, species diverse, abundantly available, pollutions negligible, and environmental friendly. This special issue, entitled Developments in Fundamentals and Applications of Renewable Energy, consists of 45 papers, which cover wide topics as follows.