Mountains cover approximately one-quarter of the world’s surface and are home to 12 percent of the human population. By providing freshwater and other key environmental services to more than half of humanity, mountain ecosystems play a crucial role in the development of the planet and contribute significantly to the well-being of human societies. This booklet summarizes state-of-the-art information on the characteristics of and threats to mountain ecosystems, the environmental services they provide and the impacts of climate change; it explains approaches to sustainable mountain development, including natural resource management, economic opportunities, and mountain policies and governance; and it describes the way forward and provides recommendations for addressing sustainable mountain development at the global and local levels. The booklet is addressed primarily to those policy- and decisionmakers who are responsible for finding a balance between socioeconomic development and environmental conservation thrusts. It shows that sustainable mountain development plays a fundamental role in addressing current global challenges, and therefore requires and deserves a prominent place on the international agenda.