In this chapter, I will summarize a number of features of mountain ecosystems that apply globally. After providing a brief statement on mountain definitions and some mountain statistics, I will recall the major climatic and atmospheric drivers and how their action is modified by geomorphology and type of vegetation. Mountains host a much higher diversity of plant species than expected from land area and the demanding life conditions. Many of the species are regional endemics, and land use, including plant collection for medical applications, is likely to threaten mountain biodiversity more than climatic change. Mountain biomes, the ones above the climatic treeline in particular, are less vulnerable to climatic change than most other biomes. It is the human population in mountains, their infrastructure, and well-being that is vulnerable to climatic change, especially when hydrological regime changes take place.