A critical overview of renewable energy is provided, including descriptions of renewable energy sources, technologies, assessments, comparisons and planning as well as energy technologies that facilitate renewable energy sources. The renewable energy types considered include solar, wind, geothermal, bioenergy and waste-derived energy, ocean thermal energy, tidal, wave and hydraulic. Also covered for contextual and broader purposes are energy systems more generally and their sustainability. In addition, recent research on new renewable energy sources as well as important recent developments in renewable energy are considered.