Summer temperatures in California are projected to increase more rapidly than previously expected, accompanied by longer, more frequent, and more severe extreme heat conditions. HadCM3 and PCM climate projections driven by the IPCC SRES emissions scenarios show summer average temperature increases of 2-5oC under the lower B1 scenario and 4-8oC under the higher A1fi scenario. As heat waves become longer and more frequent, projections for five major metropolitan areas (Los Angeles, Riverside/San Bernardino, San Francisco, Sacramento, and Fresno) show these could experience heat wave conditions on ~15 to 40 more days in the 2050s than during the 1990s. By the 2090s, the increase in heatwave days rises to 30-50 under B1 and 70-100 additional days under A1fi. Heat waves are also projected to become more intense, with higher temperatures sustained over longer periods. For an increasingly urbanized population, extreme heat waves create a significant risk of adverse health effects and heat-related mortality that is also greater than previously thought. The most severe and persistent heat conditions are projected for inland locations that already experience relatively frequent extreme heat conditions, such as Fresno and Sacramento. However, the human health impacts of increasing heat could be most serious in locations that are currently relatively cool, such as San Francisco and Los Angeles.