Tropospheric ozone (O3) is a potent threat to food security. In recent years, increasing evidence suggests that ozone can have large effects on the growth and functioning of grassland species, although the responses of grassland to ozone are complex and difficult to predict. Given the global ecological and economic importance of grasslands, there is a continuing need to research the effects of ozone on grassland and pasture. In this PhD project, the impacts of ozone on temperate managed pasture were investigated, primarily through a series of ozone-exposure experiments. Elevated ozone (seasonal mean concentration 30-67ppb) was found to have large impacts on pasture vegetation, including injury and premature senescence, reduced biomass production, declining production and yield, and impacts on forage quality. Ozone impacts on nodulation and nitrogen (N)-fixation in legumes have not extensively been investigated, but could have important consequences for pasture sustainability, with predicted reductions in the root nodule biomass of clovers (Trifolium spp.) of 12% or more in UK pasture. Ozone impacts on below ground biomass displayed a strong interaction with cutting, and intensive grazing could disproportionately increase the sensitivity of N-fixation in pasture to the effects of ozone. Flux-based ozone doseresponse relationships are constructed for biomass, total injury rates, N-fixation and forage quality in high-sugar ryegrass (HSG) pasture mesocosms, which could assist in the development of new critical levels for the protection of pasture vegetation.