This publication says that climate change is a reality today, and some of the best evidence such as melting gla¬ciers comes from mountain areas. Many scientists believe that the changes occurring in mountain ecosystems may provide an early glimpse of what could come to pass in lowland environments, and that mountains thus act as early warning systems. Mountains exist in many regions of the world. They occupy very different posi¬tions on the globe and they differ in shape, extension, altitude, vegetation cover, and climate regime. They will therefore be affected differently by climate change. However, they share some common features relating to climate change: Firstly, mountain areas have a marked and complex topography and so their cli¬mates vary considerably over short distances (Figure 1.1). Climate change projec¬tions are therefore difficult to make. Unfortunately, reliable long-term and high-altitude records of mountain climate which allow verification of regional climate models exist for only a very few areas such as the European Alps.