The critical zone sustains most terrestrial life on the planet and anthropogenic changes to the biogeochemical reservoirs and fluxes will ultimately alter terrestrial systems in unprecedented ways. The National Research Council (2001) defines the Critical Zone as the "heterogeneous, near surface environment in which complex interactions involving rock, soil, water, air, and living organisms regulate the natural habitat and determine the availability of life-sustaining resources". The critical zone can be visualized as extending from the top of the canopy to the bottom of the groundwater aquifer. The central focus is the soil, which evolves via complex biogeochemical feedbacks in response to climatic, tectonic and anthropogenic forcing at a range of spatial and temporal scales. In addition, water can be seen as the central threadthat links the atmospheric system with the lithospheric system, the hydrological system, the cryogenic system, the biological system and the human system. Ultimately, the variability of the critical zone can be described in terms of process regimes (or soil states) that develop in response to the local combination of biogeochemical processes, and the climatic and geologic framework. As a summary of critical zone research and outcomes from the NSF funded Critical Zone Observatories, the book will be an invaluable resource for undergraduate and graduate courses, as well as for researchers developing cutting-edge proposals. It provides a process-based description of the critical zone in a wide range of environments with a specific focus on the important linkages that exist amongst the processes with each critical zone.