The objective of this chapter is to explore how flow variability affects SGW interactions and thereby functions of the fluvial ecosystem along the hyporheic corridor' from head waters to river mouth (Stanford and Ward, 1993). The chapter begins by defining fluvial ecosystems in terms of two aspects of their structure and functionbiogeochemical processes and biotic structurethat are expected to respond to flow variability and SGW exchange. Data from a field experiment serve to illustrate dimensions and variability of SGW exchange at one location in space and time. The range of space and time scales over which SGW interactions occur is then considered in a case study. This analysis takes the form of a cross-continental comparison of river basins in humid (Europe) and arid/semi-arid (southwestern USA) settings, generalizing the dynamics of SGW exchange to contrasting environments as recommended by Poff et al. (2006). Finally, the relationship between patterns of flow variability and biotic processes is explored, considering important biological and anthropogenic influences on SGW exchange at different spatio-temporal scales within the fluvial ecosystems.