Book excerpt: Volcanoes release gases to the atmosphere both during and between eruptive phases. Primary and secondary processes occurring within the mantle and crust control the gases’ chemical and isotopic compositions as well as their emission rates. Therefore by measuring these gases a wealth of scientific information concerning the source and fate of these fluids is provided. Fluid geochemistry has been highly useful in advancing both our fundamental scientific understanding and procedures for operational volcano monitoring and eruption forecasting. Gases from low-to-high temperature fumaroles and those diffusively released through the soils of volcanic flanks are investigated using various sampling and measurement techniques. Furthermore, a variety of remote sensing methods are applied at relatively great distances from the source to gather major gas composition and flux data for volcanic plumes using ground based, airborne (including UAV) and space borne platforms. The acquired data have advanced science in a number of key ways: • firstly, with parallel thermodynamical modelling to advance our capacity to interpret acquired degassing data; • secondly, through improved constraints on budgets for volcanically mediated geochemical cycling, particularly via regional subduction processes; • thirdly, through improved constraints on the effects of volcanic gases on atmospheric composition, chemistry and radiative transfer, particularly in terms of halogen chemistry, volcanogenic climate change and impacts on human health; • fourthly, there has been a growing body of work focused on combining degassing data with contemporaneous geophysical data and studies on conduit fluid dynamics to advance our understanding of how subterranean gas flow mediates activity at the surface; • and fifthly, there have been considerable advances in the methods themselves, used to make the gas measurements, in particular in terms of extractive sampling (e.g., using MultiGAS units, mass spectrometry, spectroscopic isotope measurement approaches and diffusive denuder sampling) and remote sensing approaches (e.g., DOAS, UV cameras and other imaging techniques, LIDAR and FT)