The more plant species grow together, the higher the yield. Higher plant species richness may also increase drought resistance, i.e. yield preservation during droughts. However, the mechanisms underlying these positive biodiversity effects are still debated. I this thesis, I investigated whether diversity in root characteristics or ‘traits’, such as rooting depth, can explain increased yield, or ‘overyielding’, and drought resistance in grassland plant communities. My results show that, although diversity in roots traits could not explain overyielding, root traits – and deep rooting in particular – were important: communities with deep-rooting species performed better than expected. Further, overyielding of deep-rooting species in diverse communities increased when growing with shallow-rooting neighbours, supporting the importance of rooting depth diversity. This research shows that rooting strategies play an important role in the positive effect of plant diversity on yield production. We found no evidence that plant species richness or root trait diversity increased drought resistance, perhaps due to extremeness of the drought treatment. On the contrary, drought decreased overyielding. Future biodiversity research at different drought intensity levels may further increase our insights on how biodiversity affect the drought resistance of our grasslands.