Doctor of Philosophy / Department of Geography / John A. Harrington Jr / Extreme heat is a hazard that is capable of causing economic problems and potentially high mortality rates across several regions simultaneously. This dissertation was designed to provide a better understanding of how often and where heat waves occur within the United States. The research design assessed all places equally in order to evaluate geographic variations in the character of heat waves. In order to simplify the variety of extreme heat events that occur, this research developed two classifications; one for accumulated daily heat stress and a second for extended periods of extreme conditions (heat waves). Both new classification systems were designed to objectively categorize individual events using a scale from 1 (minor) to 5 (extreme). The heat wave classification system was applied to 70 locations for years 1980-2001 to determine the frequency, magnitude, and duration of daily heat stress events and heat waves. Hourly temperature and humidity data were used to determine heat index values, which were accumulated to provide the daily heat intensity measurement. Major findings from this research include: how heat stress distribution is influenced by topographical relief variations as well as latitude; daily heat stress classifications during an event were typically not in an intensify-then-weaken progression; Category 1 heat waves were the most frequent overall followed by Category 2 and Category 3 heat waves, however Category 5 events outnumbered Category 4 events over the temporal period of this study; and heat stress days/heat waves occurred most frequently in the Southeast, with the fewest occurring in the Northwest. The classification was also used to illustrate the extent and magnitude of the 1995 heat wave that caused high human mortality in the Midwest. Results from this research are presented in maps and tables to provide a detailed insight on the characteristics of heat stress throughout the United States as a function of the exposure component of hazard vulnerability.