The book Climate Change and Rocky Mountain Ecosystems was compiled by more than 100 scientists and resource managers who worked together for two years to provide a technical assessment of the effects of climatic variability and change on natural and cultural resources, based on best available science, including new analyses obtained through modeling and synthesis of existing data. This information from the nation's largest climate change vulnerability assessment to date is now being used by federal agencies and others to ensure long-term sustainability in resource conditions. Key findings include: Increased air temperature and declining snowpack are already causing changes in streams and aquatic ecosystems. Cold-water fish species will be one of the first groups of organisms to be affected by a warming climate, because of increased stream temperatures. Increased disturbances, primarily wildfires and insect outbreaks, will be the primary instigator of change in terrestrial ecosystems, leading to altered species distribution and abundance, younger forest structures, and lower carbon storage. Warm-weather recreation activities will increase in a warmer climate with longer shoulder seasons (spring, autumn), whereas winter recreation will decrease because of reduced snowpack. Although temperature is increasing, and some ecological effects of climate change have already been observed, bigger effects will be realized in the mid-to-late 21st century, especially in lower elevation aquatic ecosystems and in dry forest ecosystems and rangelands. Most of these changes will not be preventable, although adaptation options summarized in the book can be used to reduce negative biological and socioeconomic outcomes, assisting the transition of ecosystems and organizations to new conditions.