The Federal Government has sponsored job training for the unemployed and economically disadvantaged for more than three decades. The National Job Training Partnership Act (JTPA) study is the first evaluation of a major ongoing national program that uses a random assignment experimental design in which only program experience divides participants from nonparticipants. The JTPA analysis was based on data for more than 20,000 JTPA applicants in 16 local programs across the United States. The 16 local programs represent a broad range of programs, participants, and labor markets. Comparison of earnings, employment, and welfare receipt of JTPA participants and those who did not participate allows the estimation of JTPA impact and of what has worked and what has not. The report is divided into these sections: (1) "Background: JTPA Title II Year-Round Programs, Previous Research, and the National JTPA Study"; (2) "The 30-Month Impact and Benefit-Cost Analyses"; (3) "Study Sites, 30-Month Earnings Sample and Patterns of JTPA Enrollment and Service Receipt"; (4) "Program Impacts on the Target Groups"; (5) "Program Impacts on the Earnings of Subgroups"; (6) "Benefits and Costs of Title II Year-Round Programs"; and (7) "Interpretation and Policy Implications of the Findings." Appendixes present data sources and estimation methods. (Contains 76 exhibits, 4 appendix exhibits, and 80 references.) (SLD)