This book discusses geochemical evolution of the thermal waters and thermal gases in terms of the current volcano-tectonic setting and associated geological framework that makes the region very important to the geothermal scientific community. The book explains, in a didactic way, the possible applications, depending on local conditions and scales, and it presents new and stimulating ideas for future developments of this energy source. Additionally, the book discusses the role(s) of possible physicochemical processes in deep hydrothermal systems, the volatile provenance and relative contributions of mantle and crustal components to total volatile inventories. It provides the reader with a thorough understanding of the geothermal system of the Aegean Regions and identifies the most suitable solutions for specific tasks and needs at other places of the world.