In the 1996-1997 school year, the Chicago Public Schools (CPS), Illinois, began a national trend when it included a required summer program, Summer Bridge, as a central component of its efforts to end social promotion. More than 21,000 students in grades 3, 6, and 8, the promotional gate grades, have attended Summer Bridge each year, making it one of the largest and most sustained summer programs in the United States. A rigorous and careful evaluation as conducted of the Summer Bridge Program using a diverse data set that included the analysis of the achievement of all Summer Bridge participants, surveys and interviews of participating teachers and students, and in-depth classroom observations in 12 schools. Third and sixth graders received 90 hours of instruction each summer; eight graders attend 4 hours a day for 7 weeks for 140 hours. Students are taught by regular CPS teachers in small classes, usually at their own schools. CPS requires students who score below a given test score on the Iowa Test of Basic Skills to attend Summer Bridge. Among the important findings of the evaluation is that Summer Bridge has been effective in the short term in producing test score gains, especially for sixth and eight graders. The gains produced were relatively uniform across demographic and achievement groups. Students were extremely positive about their experiences in Summer Bridge. Whether teachers knew their students before Summer Bridge was an important predictor of test score increases and teacher practice. Higher achieving schools ran more effective Summer Bridge programs, and students whose teachers spent more time individualizing the curriculum and working with students outside of class had greater learning gains than those in other classrooms. Findings show that summer programs may be a useful intervention for students who are behind, but they are not a substitute effective instruction during the school year. The report includes commentaries by Geoffrey D. Borman, Jeffrey C. Valentine, Harris M. Cooper, John Poortz, and William H. Clune. Eleven appendices contain supplemental information. (Contains 59 figures, 53 endnotes, and 54 references.) (SLD)