Almost anyone, if asked to define drylands, would mention the scarce and variable precipitation and the consequently sparse vegetation cover typical of these environments. Dryland climates play a crucial role in determining the hydrologic conditions of soils and streams, the composition and spatial organization of vegetation, the rate of biogeochemical cycles, and the dominant geomorphic processes. What, more specifically, are the physical and biotic mechanisms controlling the effect of climate on ecosystem dynamics and Earth surface processes in dryland regions? How does climate, in turn, respond to changes in land surface conditions? The answer to these questions requires not only a multiscale analysis of atmospheric processes (from microclimate to the global circulation) but also the study of hydrologic, ecologic, and geomorphic dynamics.