This book includes innovative gas-geothermometers and geobarometers, which are urgently needed to estimate the increasingly higher temperatures and pressures present at depth below the Solfatara volcano, owing to its on-going unrest. Therefore, in this book, new gas geoindicators, applicable up to ca. 1000 C and 3 kbar, have been implemented and applied to Solfatara fluids. The innovations of this book include: methane, having a sluggish behavior, was treated separately from fast-reacting carbon monoxide; deviations from the ideal gas behavior were considered; the effects of reaction kinetics were taken into account. This was possible because a dataset including many geochemical parameters and extending from 1983 to 2020 with a good sampling frequency is available for Solfatara, making it a case history probably unique worldwide. Nevertheless, the gas geoindicators described in this book can be applied to other similar systems. Thus, this book is of interest to many scientists studying gas geochemistry, geothermometry, and geobarometry for volcanic surveillance and the mitigation of the volcanic risk.