The threat to human health associated with the use of antibiotic and chemical feed additives has prompted legislation in the EU to ban antimicrobial growth promoters (AGP), and has accelerated investigations into plants and their extracts as feed additives. The broad potential of plants and their extracts to replace AGP is illustrated by the progress of an EC Framework 5 project, ‘Rumen-up’ and its Framework 6 successor, ‘Replace’. The Rumen-up project began with a targetted collection of 500 European plants and their extracts, and partners tested their effects on ruminal proteolysis, protozoa, methanogenesis and lactate production. A success rate of about 5% in terms of positive hits illustrated that phytochemicals have great potential as ‘natural’ manipulators of rumen fermentation, to the potential benefit of the farmer and the environment. Some of the positive samples exerted their effect via their essential oils or saponins content. The mode of action of these phytochemicals is at least partially understood. Dietary inclusion of a commercial blend of essential oil compounds caused significantly decreased NH3 production from amino acids in ruminal fluid taken from sheep and cattle. This effect was mediated partly by effects on ammonia-producing bacteria and on the protein and starch fermenter, Ruminobacter amylophilus. Saponins-containing plants and their extracts suppress the bacteriolytic activity of rumen ciliate protozoa, thereby enhancing total microbial protein flow from the rumen. The effects of some saponins are transient, because they are hydrolysed by bacteria to their corresponding sapogenin aglycones, which are much less toxic to protozoa. Saponins also have selective antibacterial effects which may prove useful in, for example, controlling starch digestion. The Rumen-up project also highlighted potentially useful plants which had a benefit that could not be explained by our present knowledge of the effects of phytochemicals on ruminal microorganisms.