The potential for global warming and what effect it may have on weather and water resources in California and the West are explored in this issue of Western Water. Although no absolutes exist, most scientists have predicted a future of warmer temperatures and – globally – higher amounts of precipitation. Regional changes, however, may be quite different and in the West, there are some predictions that even as snowfall declines, rain will increase. Methods of climate prediction, background information on the West’s natural climate variability and an overview of the conclusions of leading scientists and major studies are included in the article, as is a section on a different approach to predicting future climate – looking at the past geologic record. Sidebars on possible climate change scenarios in the Colorado and Sacramento-San Joaquin river basins also are included.