This report presents the results and conclusions from the ambient air monitoring conducted as part of the 1999/2000 Urban Air Toxics Monitoring Program (UATMP)a program designed to characterize the magnitude and composition of potentially toxic air pollution in, or near, urban locations. The 1999/2000 UATMP included 15 monitoring stations that collected 24-hour air samples, typically on a 6- or 12-day schedule. Six sites analyzed ambient air samples for concentrations of 59 volatile organic compounds (VOC) and 16 carbonyl compounds. Ten sites also analyzed for 80 speciated nonmethane organic compounds (SNMOC). One site analyzed for the VOC, carbonyl compounds, and 92 semivolatile compounds (SVOC). Overall, nearly 28,000 ambient air concentrations were measured during the 1999/2000 UATMP. The summary presented in this report uses various graphical, numerical, and statistical analyses to put the vast amount of ambient air monitoring data collected into perspective. Not surprisingly, the ambient air concentrations measured during the program varied significantly from city to city and from season to season. This report describes and interprets these spatial and temporal variations separately for halogenated hydrocarbons, hydrocarbons, polars, and carbonyls.