This chapter focuses on the variability of soil food webs. Information on temporal and spatial heterogeneity is crucial for understanding how soil food webs and soil biodiversity affect key ecosystem processes. Surprisingly little is known about spatial and temporal variability in soil communities and food webs. Variability in food webs has most often been studied in aquatic ecosystems. Sensitivity analyses of soil food web models have shown that small changes in the biomass of particular groups-for example, basal organisms or certain predators-may have a marked and disproportionate effect on soil processes, such as decomposition and mineralization of nutrients. Variability in species composition may not be clearly related to food web variability and ecosystem functioning, because of species redundancy and complementarity within functional groups, and because the scales of spatio-temporal changes in the abundance or biomass of many functional groups are very small compared to the scales at which ecosystem functioning is relevant to the study.