Our understanding of the role of fluids in the genesis of metamorphic rocks is rapidly expanding due to the many new approaches to the analysis of metamorphic rocks and modeling of metamorphic processes. It is apparent from the papers in volume 5 of Springer-Verlag's Advances in Physical Geochemistry Series that metamorphic fluids have a far more active role in mass and energy transfer than was previously envisaged. This volume contains eight papers on the physical and chemical characteristics of metamorphic fluids, including their generation, their migration, and their reactions with a variety of rock types during metamorphism.