This study examined whether the Chicago Annenberg Challenge promoted improvement in schools it supported and in student achievement and other outcomes; factors that might explain improvement or lack thereof among Annenberg schools; and what could be learned from the Challenge's experiences. It highlighted the period between 1996-1997 through 2000-2001. Results suggest that among the schools it supported, the Challenge had little impact on school improvement and student outcomes, with no statistically significant differences between Annenberg and non-Annenberg schools in rates of achievement gain, classroom behavior, student self-efficacy, and social competence. The study also examined trends in school improvement among a small group of "Breakthrough Schools," which received special financial and professional support from the Challenge between 1990-2001, a time during which the Challenge began withdrawing funds from the other Annenberg schools. Overall, Breakthrough schools began to develop in ways that distinguished them from other Annenberg schools and sustained or strengthened aspects of teacher professional community, school leadership, and relational trust while other Annenberg schools did not. Factors distinguishing strong and weak schools included cultivation of strong, distributive leadership and use of an array of complementary, reinforcing strategies. Appendixes contain research methodology and results. (Contains 59 figures, 27 tables, and 131 endnotes.) (SM)