Frontiers in Geofluids is a collection of invited papers chosen to highlight recent developments in our understanding of geological fluids in different parts of the Earth, and published to mark the first ten years of publication of the journal Geofluids. The scope of the volume ranges from the fundamental properties of fluids and the phase relationships of fluids encountered in nature, to case studies of the role of fluids in natural processes. New developments in analytical and theoretical approaches to understanding fluid compositions, fluid properties, and geological fluid dynamics across a wide range of environments are included. A recurrent theme of research published in Geofluids is the way in which similar approaches can be applied to geological fluids in very different settings and this is reflected in the diverse range of applications of fluid studies that are included here. They include deep groundwater flow, hydrocarbons in faulted sedimentary basins, hydrothermal ores, and multiphase flow in mid-ocean ridge systems. Other topics covered are geothermal waters, crustal metamorphism, and fluids in magmatic systems. The book will be of great interest to researchers and students interested in crustal and mantle fluids of all sorts.