THE PRINCIPAL OBJECTIVE OF THIS STUDY WAS TO LEARN ABOUT THE ROLE OF PRETRIAL DETENTION IN THE AMERICAN CRIMINAL JUSTICE SYSTEM. IN ORDER TO STUDY PRETRIAL DETENTION IT WAS NECESSARY TO EXAMINE THE DECISION PROCESS CONCERNING BAIL THAT GOVERNS ITS USE. ANALYSIS OF LEGAL GUIDELINES WAS CONDUCTED TO DISCOVER THE PURPOSE OF THE BAIL DECISION AND THE CRITERIA ON WHICH IT IS BASED. IT WAS CONCLUDED THAT TWO COMPETING VIEWS OF THE BAIL FUNCTION ARE FREQUENTLY RECOGNIZED. THE TWO VIEWPOINTS ARE THE DANGEROUSNESS-DETECTING VIEW AND THE COURT APPEARANCE-GUARANTEEING VIEW. CONSIDERABLE CONFUSION CHARACTERIZES THE DECISION CRITERIA RECOMMENDED IN THE VARIOUS, EXISTING GUIDELINES. DATA GATHERED FROM A NATIONAL SURVEY OF JAILS WERE ANALYZED TO OBTAIN A PICTURE OF THE KINDS OF DEFENDANTS TYPICALLY DETAINED IN THE NATION'S JAILS. NATIONWIDE, DETAINED DEFENDANTS APPEARED TO BE DISPROPORTIONATELY YOUNG, MALE, BLACK, AND LOW INCOME OR UNEMPLOYED, WITH MANY WHO WERE IN JAIL DURING PREVIOUS YEARS UNDER DIFFERENT CIRCUMSTANCES. A MAJOR COMPONENT OF THIS STUDY INVOLVED RESEARCH CONDUCTED IN PHILADELPHIA, PA., USING A LARGE COHORT OF DEFENDANTS APPEARING FOR THE FIRST TIME IN THE FALL OF 1975. THE BAIL DECISION WAS ANALYZED EMPIRICALLY USING A DECISION MODEL HELPING TO SPECIFY THE FACTORS VIEWED AS IMPORTANT IN DIFFERENT ASPECTS OF THE BAIL DECISION. SIMILARLY, A COMPREHENSIVE ANALYSIS OF PRETRIAL CUSTODY WAS CONDUCTED, WITH SPECIAL EMPHASIS ON THE PREDICTION OF DIFFERENTIAL RATES OF DETENTION FOR A NUMBER OF EMPIRICALLY DERIVED TYPES OF DEFENDANTS. IN A SPECIAL COMPONENT OF THE STUDY, THE QUESTION OF WHETHER PRETRIAL CUSTODY HAS AN INFLUENCE ON THE LATER OUTCOMES OF DEFENDANTS' CASES WAS CONSIDERED, AND THEN ASSESSED. TABLES AND A LIST OF REFERENCES ARE INCLUDED. (AUTHOR ABSTRACT MODIFIED--MLC)