The first component of the study empirically addressed the 'selectivity' of pretrial detention through two sets of data. One derived from a random sample of 463 defendants detained in the Philadelphia jails prior to trial on a typical day (November 13, 1980). The second set of data was obtained from a sample of 462 defendants selected by court order for emergency release from detention as a result of overcrowding litigation (Jackson v. Hendrick). Using these data, the study examined the characteristics of those typically detained prior to trial and drew inferences about the selectivity of detention practices. Further inferences about the predictive effectiveness of detention were examined in the 'natural' experiment created by the expedited release of defendants who would otherwise have remained in detention. The second component of the study tested the accuracy of a predictive classification recently developed through a study of released Philadelphia defendants. As applied to both study samples, this device provided a singular analytic framework that yielded evidence relating to the function of pretrial detention and its selectivity, as well as to the general utility of predictive classification in bail and detention. Defendants released under the 'Jackson' decision were followed for 90 days. Forty-two percent of the sample were recorded as willfully failing to appear (FTA) in court at least once. Twenty-eight percent were arrested for crimes during pretrial release, with about 24 percent being for serious crimes against the person. In the representative cohort, about 12 percent recorded willful FTA's, and 17 percent were rearrested for crimes during a followup period of 120 days. These findings indicate that detention in Philadelphia is selective. Also, the instrument was found to predict poor risks with a significant degree of accuracy. Although Philadelphia is effective in deciding which defendants should be detained prior to trial, the classification instrument indicated that a significant minority of low-risk defendants was still detained. Graphic and tabular data from the study are provided along with 67 footnotes.