This paper summarizes the results of the comprehensive first flush characterization study performed on three highly urbanized highway sites in Los Angeles, California. The study was performed from 2000 through 2005 and a total of 97 storm events were monitored ranging from 0.5 to 137 mm with an average rainfall of about 25.5 mm. First flush characterization was performed based on contaminant concentration and mass loading. To determine the first flush effect, multiple grab samples were obtained throughout the storm event with an emphasis on collecting additional grab samples during the first hour of storm event. Topics presented and discussed in this paper include: (1) meaningful definition of first flush characterization, (2) strategic method of collecting first flush sampling, (3) uniform method and interpreting first flush results, and (4) the implication of first flush results for urban stormwater runoff management.