Hydrothermal processes on Earth have played an important role in the evolution of our planet. These processes link the lithosphere, hydrosphere and biosphere in continuously evolving dynamic systems. Terrestrial hydrothermal processes have been active since water condensed to form the hydrosphere, most probably from about 4.4 Ga. The circulation of hot aqueous solutions, or hydrothermal systems, at and below the Earth's surface, is driven by magmatic heat. Hydrothermal systems form beneath the oceans (e. g. spreading centres, oceanic plateaux), in lakes, intracontinental rifts, continental margins and magmatic arcs. Hydrothermal fluids can be juvenile-magmatic or derived from seawater, metamorphic, meteoric, connate waters or a mix of two or more of these.The interaction of hydrothermal fluids with wallrocks and/or the hydrosphere and changes in their composition through time and space, contribute to the formation of a wide range of mineral deposit types and associated wallrock alteration. On Earth, sites of hydrothermal activity support, both at surface and in the subsurface, varied ecosystems based on a range of chemotrophic microorganisms. An intriguing question is whether primitive life is still being created in present-day hydrothermal systems, or did it occur only in the ancient geological past? This volume will be of interest to scientists and researchers in Geo and Life Sciences departments, as well as to professionals and scientists involved in mining and mineral exploration.