To establish usage rates for scientific evidence and to determine the effects of this evidence on case outcome, a random sampling of felony cases in six jurisdictions across the country were studied. The samples were taken from 1976, 1978, and 1981. A mail survey of all U.S. crime laboratories profiled the scope and sophistication of laboratory services. Prosecutors and defense attorneys in the six study sites were interviewed to ascertain their perceptions of the importance of forensic evidence relative to other evidence types. A sample of prosecutors at one site assessed hypothetical cases regarding the relative value of various evidence types. Several hundred jurors who had just returned verdicts in felony trials were interviewed about the impact of various types of evidence on their verdicts. Laboratory reports of evidence were typically found in about one-quarter to one-third of felony case files; more than half pertained to the identification of controlled substances. With the exception of drug offenses, forensic evidence was unimportant in the charging decision. Forensic testimony was used in a small percentage of trials, although prosecutors and jurors believe it to be important. In two sites, the presence of laboratory reports led to higher incarceration rates in sentencing. Policy implications are discussed. Tabular data and 61 references.