Fifteen highway construction sites were monitored by the California Department of Transportation (Caltrans) to assess the water quality of storm-water runoff from the sites. Caltrans conducted a study to generate sufficient water quality data to further develop management strategies and evaluate existing best management practices. A wide range of construction sites was selected for monitoring throughout the state. Both flow-paced composite and single-grab samples were collected and analyzed at these sites for a total of 72 station-storm events during the 1998-1999 and 1999-2000 wet seasons. Results obtained during the 2-year characterization study indicate the following: (a) Caltrans construction-site runoff constituent concentrations detected during this study are less than typical Caltrans and non-Caltrans highway runoff constituent concentrations, with the exception of total chromium, total nickel, total phosphorus, total suspended solids (TSS), and turbidity. (b) The concentrations of TSS and turbidity likely are due to the disturbed soils present at most construction sites. (c) The origin of the high concentrations of total chromium, total nickel, and total phosphorus concentrations is unknown. Concentrations of these constituents varied between sites, so it is possible that site-specific soils and vegetative conditions contributed to the concentrations of these constituents. (d) A correlation (R-squared values greater than 0.5) was observed between TSS runoff concentrations and particulate runoff concentrations of chromium, copper, and zinc, indicating that minimizing particulate matter may reduce total metals concentrations.