The Federal Highway Administration and State transportation agencies have the responsibility of determining and minimizing the effects of highway runoff on water quality; therefore, they have been conducting an extensive program of water-quality monitoring and research during the last 25 years. The objectives and monitoring goals of highway runoff studies have been diverse, because the highway community must address many different questions about the characteristics and impacts of highway runoff. The Federal Highway Administration must establish that available data and procedures that are used to assess and predict pollutant loadings and impacts from highway stormwater runoff are valid, current, and technically supportable. This report examines criteria for evaluating water-quality data and resultant interpretations. The criteria used to determine if data are valid (useful for intended purposes), current, and technically supportable are derived from published materials from the Federal Highway Administration, the U.S. Environmental Protection Agency, the Intergovernmental Task Force on Monitoring Water Quality, the U.S. Geological Survey and from technical experts throughout the U.S. Geological Survey. Water-quality data that are documented to be meaningful, representative, complete, precise, accurate, comparable, and admissible as legal evidence will meet the scientific, engineering, and regulatory needs of highway agencies. Documentation of basic information, such as compatible monitoring objectives and program design features; metadata (when, where, and how data were collected as well as who collected and analyzed the data); ancillary information (explanatory variables and study-site characteristics); and legal requirements are needed to evaluate data. Documentation of sufficient quality-assurance and quality-control information to establish the quality and uncertainty in the data and interpretations also are needed to determine the comparability and utility of data sets for intended uses. The fact that a program's data may not meet screening criteria for a national synthesis does not mean that the data are not useful for meeting that program's objectives or that they could not be used for water-quality studies with different objectives.