Book excerpt: This volume focuses on a few renewable energy sources, viz. wind energy plus energy from water movement and natural temperature differences that in principle could provide enormous energy resources. Energy from wind has been a rapidly growing source of energy as wind turbines have grown in size and especially as wind turbines have moved offshore. Hydroelectric dams have continued to be used as energy sources particularly in developing countries. Other energy sources using water, including waves and tidal sources, are also discussed in this volume. Finally, the volume discusses differences between deep and surface ocean temperatures plus the extraction of energy from the earth's extremely large energy resource of magma deep below the surface. These latter two energy resources in particular require further development and the current book describes the latest advances coupled with pointing possible paths forward.