This century will definitively witness the replacement of current fossil fuels (i.e., oil, natural gas, and coal) with renewable energy sources. Currently, fossil fuels account for almost 80 % of the total energy consumed worldwide and, consequently, there is still a long way to walk before this replacement comes true. Renewable energy-based technologies are being improved fast and they are, little by little, reducing the development level gap with the well-known and optimized fossil technologies. This book offers a compendium of works describing these efforts on some of the most representative renewable technologies. The first five chapters are dedicated to biomass, the only renewable source of organic carbon available on Earth and thus, the only sustainable source for fuels and chemicals. Hydrogen is considered by many as the energy vector of the future. While the hydrogen technologies are in their infancy (as stressed in Chapter 6), there has been a tremendous progress in the last years on renewable hydrogen production and storage. Chapter 7 reviews the current state of one of the most interesting technologies for hydrogen production to date: photocatalytic water splitting. Photovoltaic technologies (Chapters 8 and 9), able to directly convert sunlight into electricity, are extremely important and have reached a high level of development. The discontinuous nature of renewable energies obligates to develop efficient storage systems (Chapters 10–12) such that the energy can be used when most required. Finally, Chapters 13 and 14 summarizes the current state of the electricity generating and tidal power technologies.