The personal safety and well-being of one fifth, and water supply for almost half of all people depend directly or indirectly on the functional integrity of mountain ecosystems, the key component of which is a robust vegetation cover. The green 'coat' of the world's mountains is composed of specialized plants, animals and microbes, all nested in a great variety of microhabitats. Because a single mountain may host a series of climatically different life zones over short elevational distances, mountains are hot spots of biodiversity and priority regions for conservation. With their diverse root systems, plants anchor soils on slopes and prevent erosion. Both landuse and atmospheric changes such as elevated CO2 and climatic warming affect mountain biodiversity. Sustained catchment value depends on sustained soil integrity, which in turn depends on a diverse plant cover. Whether landuse in mountains is sustainable is a question of its consequences for water yield and biodiversity. Given their dependence on mountains, lowlanders should show concern for the highlands beyond their recreational value.