This collection of chapters from the Treatise on Geochemistry describes the present and the past operation of the major Earth surface systems. Chapters on volcanic and hydrothermal systems are followed by descriptions of the geochemical cycles of carbon and sulfur, the nature of weathering, the composition of river waters, the operation of the oceans, and the evolution of sedimentary rocks. The final chapter on the behavior of volatiles in subduction zones closes the cycle that is opened in the first two chapters.