In an agricultural context where farmers and citizens have growing environmental concerns, horses – whose are more than 6 millions across Europe - have a significant role to play for land management and biodiversity conservation. We review the state of art on the effects of horse grazing on plant and fauna diversity in different types of grasslands and heathlands across Europe. Among 40 references found in the WoS in February 2020, 35 were surveys from areas of high nature value. Horses are less affected than ruminants by digestive constraints: their high voluntary intake on roughages enables them to control competitive grasses as well as reeds and sedges in wet areas. This maintains open areas and promotes the coexistence of many plant and animal species in pastures, especially when these are grazed at a lenient stocking rate. Mixed grazing with cattle appears effective for controlling shrubs that are usually avoided by horses unable to detoxify secondary metabolites from dicotyledons. Their consumption of forbs and legumes is lesser than that of ruminants, with positive consequences on flowering plants and flower-visiting insects compared to sheep grazing or no grazing. Horses with their two sets of incisors create stable short patches of grass within a matrix of taller vegetation. Increase in the structural heterogeneity of pastures favours grassland biodiversity. Under low to moderate stocking rate, mixed grazing between cattle and horses would maximize botanical diversity compared to horse or cattle grazing alone, as cattle increase diversity in patches of tall grasses avoided by horses. Horse trampling under high stocking rate could have detrimental effects on plant and animal species. In mesophile grasslands which are commonly used in horse production systems in Europe, a number of studies have improved our understanding of factors (body size, sward height, supplementation) affecting voluntary intake and feeding choices by horses. Only five references report how grazing horses impact grassland biodiversity, revealing an increase in legume abundance in stable grazed patches and some effects on insect diversity. Further research is needed in mesophile grasslands to propose management options that jointly meet production goals and ecosystem integrity.