The U.S. National Research Council has defined Earth's Critical Zone as terrestrial surface ecosystems including soil and the underlying vadose zone. It is a region characterized by complex interdependence of climate, hydrology, biology, lithology, topography, pedology, chronology, and anthropogenic forcing. The Critical Zone concept provides an appealing framework for funding agencies to support integrated studies of Earth surface processes. An integrated network for conceptualizing and modeling, Earth's Critical Zone as a whole is in the early stages of development, but it is clear that it will require input from many basic and applied disciplines. Hydropedology, the science of the behavior of water in contact with mineral and biological material in the Critical Zone, will be an important contributor to the Critical Zone research. The U.S. National Science Foundation's recommendation to focus on water as a unifying theme for research on complex environmental systems is, separate from, but consistent with the Critical Zone concept. Here we explicitly link the two ideas. We suggest that the four dimensions of water issues (quantity, quality, distribution including extreme events, and social-economy) can be addressed using the framework of Earth's Critical Zone. Hydropedology provides a powerful tool to explore the Critical Zone by integrating hydrologic, pedologic, and geomorphic processes. Water movement links pedons to landscapes to watersheds, even as it facilitates fluxes of water, energy, chemical elements, and supports terrestrial primary production and respiration. In fact, terrestrial hydrological and biogeochemical cycles are inseparable – water provides most of the transport power. In this paper, we illustrate the benefit of integrating hydrologic, pedologic, and geomorphic expertise in understanding the Critical Zone. These examples are: (1) soils' role in hillslope and watershed hydrology; (2) hydrology's role in pedogenesis and hydromorphism; (3) hydrogeomorphology and natural hazards; (4) paleopedology, paleohydrology, and paleogeomorphology; and (5) climate change and terrestrial carbon cycle. In all of these examples, complex landscape-soil-water dynamic interactions are an essential component, which is the foundation for the concepts of the Critical Zone and hydropedology.