Meteorological and climatic processes in mountain regions play a key role in many environmental systems, in particular the quantity and quality of water that influences both aquatic ecosystems and economic systems often far beyond the boundaries of the mountains themselves.This paper will provide a general overview of some of the particular characteristics of mountain weather and climate, to highlight some of the unique atmospheric features that are associated with regions of complex topography. The second part of the paper will focus upon characteristics of climate and climatic change in the European Alps, a region with a wealth of high quality data that allows an assessment on how climate and dependent environmental systems have evolved in the course of the 20th century and how alpine climate may undergo further changes to ‘‘global warming'' in the 21st century, as the atmosphere responds to increasing levels of greenhouse gases that are expected in coming decades. Introduction Significant orographic features occupy close to 25% of continental surfaces (Kapos et al., 2000) and, although only about 26% of the world's population resides within mountains or in the foothills of the mountains (Meybeck et al., 2001), mountain-based resources indirectly provide sustenance for over half. Moreover, 40% of global population lives in the watersheds of rivers originating in various mountains of the world. Although mountains differ considerably from one region to another, one common characteristic is the complexity of their topography. Orographic features include some of the sharpest gradients found in continental areas. Related characteristics include rapid and systematic changes in climatic parameters, in particular temperature and precipitation, over very