High mountains (i.e., mountains that reach above the climatic treeline) are regions where many interests converge. Their treeless alpine landscapes and ecosystems are key areas for biodiversity, they act as water sources and reservoirs, and they are cultural and religious icons. Yet, mountain environments are threatened by global stressors such as land use impacts and anthropogenic climate change, including associated species redistributions and invasions. High mountains are warming faster than lower elevations. The number of frost days is declining, glaciers are retreating, and snow is remaining for shorter periods, while CO2 partial pressure is increasing. All of these factors affect the way in which ecosystems prosper or degrade. Thanks to the compression of thermal belts and to topographic ruggedness that favors habitat heterogeneity, mountains have a high diversity of biotic communities and species richness at the landscape level. In tropical to temperature regions, high mountains are biogeographically much like islands. With small habitat areas, species tend to be distributed patchily, with populations evolving independently from those on other isolated summits. Although high mountain areas strongly differ in size, geological age, bedrock, glacial history, solar radiation, precipitation patterns, wind exposure, length of growing season, and biotic features, they are all governed by low-temperature conditions. Combined with their distribution over all climate zones on Earth, mountain habitats and their biota, therefore, represent an excellent natural indicator system for tracing the ecological impacts of global climate change. As temperatures rise, plants and animals migrate upward (and poleward). Plant and animal populations on small, isolated mountains have nowhere to go if climates warm and push them upslope. On the other hand, habitat heterogeneity may buffer against biodiversity losses by providing a multitude of potential refugia for species which become increasingly maladapted to their present habitats. Global-scale approaches to monitor climate and biotic change in high mountains as well as modeling and experimental studies are helping explain the nature of these changes. Such studies have found that species from lower elevations are colonizing habitats on mountain summits at an accelerating pace, with five times faster rates than half a century ago. Further, repeated in situ surveys in permanent plots showed a widespread transformation of alpine plant community assemblages toward more warmth-demanding and/or less cold-adapted species. Concurrently to widespread increases in overall species richness, high-elevation plant species have declined in abundance and frequency. Strongly cold-adapted plant species may directly suffer from warmer and longer growing seasons through weak abilities to adjust respiration rates to warmer conditions. Combined effects of warming and decreasing water availability will amplify detrimental effects of climatic stresses on alpine biota. Many of the dwarf and slow-growing species, however, will be affected when taller and faster-growing species from lower elevations invade and prosper with warming in alpine environments and, thus, threaten to outcompete locally established species. Warming conditions will also encourage land use changes and upward movement of agriculture, while loss of snow is a loss to ski fields and scenic tourism.