It could be argued that the understanding of eco-hydrological processes is critical in drylands for two reasons. First, in most cases the aridity of the dry lands means that the supply of water is the dominant control on the growth and maintenance of plants. Second, most drylands are characterised by extreme variability in water availability. Plants must adapt to use this variable source. In addition, large amounts of soil erosion during storm events can remove the uppermost, relatively fertile and moisture-retaining parts of the soil profile and thus enhance the importance of moisture availability as the dominant control on dry land vegetation. In this chapter, we will explore these interrelationships and feedback mechanisms at a number of spatial and temporal scales. In doing so, we have divided the material into three major sections.