Volume 76 of Reviews in Mineralogy and Geochemistry presents an extended review of the topics conveyed in a short course on Geothermal Fluid Thermodynamics held prior to the 23rd Annual V.M. Goldschmidt Conference in Florence, Italy (August 24-25, 2013).It covers Thermodynamics of Geothermal Fluids, The Molecular-Scale Fundament of Geothermal Fluid Thermodynamics, Thermodynamics of Aqueous Species at High Temperatures and Pressures: Equations of State and Transport Theory, Mineral Solubility and Aqueous Speciation Under Hydrothermal Conditions to 300 C - The Carbonate System as an Example, Thermodynamic Modeling of Fluid-Rock Interaction at Mid-Crustal to Upper-Mantle Conditions, Speciation and Transport of Metals and Metalloids in Geological Vapors, Solution Calorimetry Under Hydrothermal Conditions, Structure and Thermodynamics of Subduction Zone Fluids from Spectroscopic Studies and Thermodynamics of Organic Transformations in Hydrothermal Fluids.