The Horse Chestnut (Aesculus hippocastanum) is a major amenity tree native to Greece and the central Balkan peninsula and planted across Europe. It has been significantly damaged by the leaf miner moth Cameraria ohridella across its entire native and introduced range; the extent of decline caused by infestation is thought to be insignificant, however, compared to the multiple threats the Pindus Mountain mixed forest ecoregion is facing. The species is threatened or likely to be extinct across most of its native range: it is Endangered in Bulgaria (Petrova and Vladimirov 2009, Bulgarian Academy of Sciences and Ministry of Environment and Water 2011), where it remains in two locations, and in Greece, where the declining population is estimated at only 259-407+ trees; it is probably Extinct in Albania. The status of the population in Macedonia is unknown, but given the small range in the country, it is likely to be small. The species occurs in protected areas in Greece and Bulgaria, including national parks/reserves and Natura 2000 sites, although mining, deforestation, tourism development and other threats still impact some national parks. Given the widespread and varied threats across its native range, the population is almost certainly suffering a continuing decline, though the overall decline has not been quantified. Although the total population size across its native range has not been estimated, it is unlikely to consist of more than 10,000 mature individuals and may even be fewer than 2,500 mature individuals. Based on the subpopulation structure in Greece and the ongoing threats across its range, all wild subpopulations are likely smaller than 1,000 individuals. At the European level, Aesculus hippocastanum is therefore assessed as Vulnerable C2a(i). It also qualifies for Vulnerable C2a(i) in its EU 27 range (Bulgaria and Greece), where the majority of the native population is found. There is likely to be immigration of propagules into its native range as it has been introduced throughout Europe, so the original category is downlisted to Near Threatened in both Europe and the EU 27. Recommended conservation measures include controlling the Cameraria ochridella leaf miner, enforcement of protection regimes in nature reserves, regulating human impacts on its habitats, and ex situ cultivation using genetic material from remaining natural populations. Research is needed on the genetic similarity between native and introduced subpopulations, to determine if introduced subpopulations likely to be the source of propagules may indeed help augment declining native populations.