This review is based on two sources. The first source is a case-series study of 140 deaths∗ identified by California county coroners and medical examiners as “heat related” (HR) from July 15 to August 1, 2006. Coroners and medical examiners provided copies of death certificates, investigation narratives, and autopsy reports with toxicology findings. The investigators (Thomas Kim, M.D. and Roger Trent, Ph.D.) consider the findings reliable but not final. This document summarizes the preliminary findings, and is not a detailed discussion of all aspects of the investigation or a detailed chronicle of all known aspects of the deaths. Analysis of this California case series continues. The second source is a literature review of 26 studies of HR morbidity and mortality from the United States, Europe, and Australia. The review is the work of Thomas Kim, M.D. and Kathleen Fitzsimmons, M.P.H. Any findings of the California study that appear to be new or different from findings in the literature are emphasized. A working hypothesis was that heat related deaths in California might differ from those in other places, especially parts of the United States and Europe with generally higher humidity and differences in demographics, activity patterns, the number of microclimates, and other factors. By looking at both past research and the California case series findings, we aimed to ground the assessment of the Draft Plan in data that are comprehensive as well as germane to California’s experience in last summer’s heat wave. This document describes the main factors in HR illness and deaths, and opportunities for prevention. All the data graphs and tables come from the California case series. At the end, we provide conclusions based on our findings about these main factors.