Arguments for and against the use of the exclusionary rule, which forbids the introduction at trial of evidence obtained illegally, have been based more on subjective perceptions and philosophical outlooks than empirical evidence on the impact of the rule. Although the primary intent of the exclusionary rule is to deter police abuse of citizen privacy rights, empirical investigations have not been able to assess the rule's utility in this regard because of researchers' inability to measure directly police search and seizure practices prior to the Supreme Court's imposition of the rule on the States in 1961. Following any eventual nationwide modification or abandonment of the rule, it might be possible to conduct an empirical 'before-after' test of the effect on police performance of altering the rule. Insight might also be gained into the effect of revising the rule if it were experimentally retained for some jurisdictions and changed for others. Also, other strategies for deterring police privacy abuses might be tested for their effectiveness. A General Accounting Office study of the impact of the exclusionary rule on Federal criminal prosecutions showed that illegal searches and seizures accounted for .4 percent of the cases U.S. attorneys declined to accept for prosecution and accounted for about .7 percent of the cases dismissed after prosecution had begun. Studies of the impact of the exclusionary rule in State and local criminal cases show a similar low percentage of cases refused or dismissed because of the exclusionary rule. Empirical evidence as yet does not provide solid ground for either advocates or opponents of the rule.