Forage and grassland legumes are rich in nitrogen and easy to digest. Their use can increase the efficiency of livestock systems, whether in pastures or as part of winter rations. Exploiting legumes can also lead to significant savings because the need for protein concentrates is reduced. Further advantages are improved animal health, reduced nitrogen pollution in animal waste, and higher milk and meat quality. These benefits are likely rooted in the presence of condensed tannins, which are found in sainfoin, for example, or of polyphenol oxidase, found in red clover. Such compounds affect how ruminants metabolise the proteins and lipids found in their forage. Condensed tannins also act as antihelminthics and help limit methane emissions. This in-depth literature review seeks to provide a detailed picture of recent related discoveries involving lucerne, white clover, red clover, and sainfoin.