This study by researchers at the University of California, Irvine, the University of Wisconsin-Madison and Policy Studies Associates, Inc. finds that regular participation in high-quality afterschool programs is linked to significant gains in standardized test scores and work habits as well as reductions in behavior problems among disadvantaged students. These gains help offset the negative impact of a lack of supervision after school. The two-year study followed almost 3,000 low-income, ethnically diverse elementary and middle school students from eight states in six major metropolitan centers and six smaller urban and rural locations. About half of the young people attended high-quality afterschool programs at their schools or in their communities. (Contains 2 exhibits.)