The study selected 51 police agencies in Illinois, which were selected to achieve wide coverage of the types of police agencies throughout the State while keeping the sample size relatively manageable for doing field interviews. Data collected through the various data-collection methods included a standardized departmental/administrative questionnaire to be completed by an administrative officer within each agency; an anonymous, self-administered questionnaire distributed to individual sworn officers within each agency; a brief closed-ended telephone interview conducted with an administrator in each department that had returned the administrative survey; and an on-site, face-to-face, semi-structured, open-ended field interview with individual officers from selected departments within the sample. For this study a pursuit was nominally defined as "an active attempt by a law enforcement officer on duty in a patrol car to apprehend one or more occupants of a moving motor vehicle, providing the driver of such vehicle is aware of the attempt and is resisting apprehension by maintaining or increasing his speed or by ignoring the law officer's attempt to stop him." The officer-reported estimates of the frequency of police pursuits are eight to nine times higher than the official administrative numbers. The estimated frequency of vehicle pursuits from officer questionnaires remained approximately seven times higher than the comparable estimate from administrative data and reports. This reveals a large "dark" figure of pursuits when relying on official or departmental information. Regarding the characteristics of pursuits, the officer surveys produced somewhat higher estimates of the number of pursuits that end in harmful outcomes than did administrative data. Police officers reported 50 percent higher rates of accidents (34.5 percent to 23.1 percent) and 100 percent higher injury rates (16.9 percent to 8.8 percent). The data sources also differed noticeably in the estimated numbers of pursuit-linked deaths. The report discusses the implications of the findings for data-collection methods for police pursuits. 21 references and 2 tables