Ecohydrology is concerned with the effects of hydrological processes on the distribution, structure, and function of ecosystems, and with the effects of biotic processes on elements of the water cycle (Nuttle, 2002). At the heart of ecohydrology is soil and soil moisture. Thus, only by understanding how the distribution and patterns of plants within the soil affect soil moisture-driven, Critical Zone processes will it be possible to decipher the Earth System. This effort begins with the root-soil-rock interface at the pore scale and extends to the vegetation-atmosphere boundary at global scales. This chapter focuses on the ecohydrology of both energy- and water-limited environments and on the feedbacks between vegetation and water across patch, hillslope, and catchment scales.