Biodiversity is thought to insure ecosystem functioning in a changing world. There is considerable theoretical and empirical evidence that declines in diversity could decrease average levels of ecosystem functioning and increase the temporal variability of productivity. Yet it remains unclear whether plant diversity also buffers ecosystems from perturbations, such as altered climate or biogeochemical cycles. To comprehensively test this hypothesis, we will synthesize data from many biodiversity studies to investigate whether ecosystem functioning changes less (higher resistance) and recovers more quickly (higher resilience) from perturbations in high- than in lowdiversity plant communities. We will consider perturbations that were either observed, such as extreme climatic events like drought, or that were experimentally imposed, such as manipulations of nutrients, water, carbon dioxide, or herbivores. We will further distinguish between the effects of press and pulse perturbations; between species- and community-level responses; between taxonomic and functional diversity; and between perturbations that increase or decrease abundances. For example, we will test whether community productivity decreases less in highthan in low-diversity communities during a drought, or under a precipitation removal treatment that was crossed with a diversity treatment. We will also synthesize and extend theory to predict the conditions under which diversity enhances or reduces resistance and resilience.