U.S. social programs, set up to address important problems, often fall short by funding specific models/strategies ("interventions") that are not effective. When evaluated in scientifically-rigorous studies, social interventions in K-12 education, job training, crime prevention, and other areas are frequently found ineffective or marginally effective. Interventions that produce sizable, sustained effects on important life outcomes tend to be the exception. The Top Tier Evidence Initiative seeks to assist policy officials in identifying interventions meeting the Congressional Top Tier evidence standard, defined in recent legislative provisions as "well-designed randomized controlled trials [showing] sizeable, sustained effects on important outcomes." This evidence summary describes and evaluates Success for All, a comprehensive school-wide reform program, primarily for high-poverty elementary schools, with a strong emphasis on early detection and prevention of reading problems before they become serious. Key program elements include: (1) daily 90-minute reading classes, each of which is formed by grouping together students of various ages who read at the same performance level; (2) a K-1 reading curriculum that focuses on language development (e.g., reading stories to students and having them re-tell), teaching students the distinct sounds that make up words (i.e. phonemic awareness), blending sounds to form words, and developing reading fluency; (3) daily one-on-one tutoring (in addition to the classes) for students needing extra help with reading; and (4) cooperative learning activities (in which students work together in teams or pairs) starting in the grade 2 reading classes. Success for All meets the Top Tier Evidence Standard, defined by recent Congressional legislation. Discussion of the study quality and a summary of the intervention's benefits and costs are also provided.