This report presents the results and conclusions from the ambient air monitoring conducted as part of the 2003 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 2003 UATMP included 53 monitoring stations that collected 24-hour air samples, typically on a 6- or 12-day schedule. Forty-eight sites analyzed ambient air samples for concentrations of 59 volatile organic compounds (VOC) and 16 carbonyl compounds. Nine sites also analyzed for 80 speciated nonmethane organic compounds (SNMOC). One site analyzed for 19 semivolatile compounds (SVOC). Nine sites analyzed metal compounds, while two sites analyzed hexavalent chromium. Overall, nearly 118,600 ambient air concentrations were measured during the 2003 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, polar compounds, and carbonyls.