This book explores water geothermometry, a highly relevant topic in the exploration and exploitation of geothermal energy. Presenting theoretical geothermometers and indicators of CO2 fugacity, it describes a rigorous new approach entirely based on thermodynamics. The book will appeal to geothermal geoscientists, especially those working in research institutions and companies around the globe. It is also of interest to students on advanced courses in applied geochemistry, water–rock interaction and other related areas.