More than 1,000 Illinois schools are implementing schoolwide positive behavior support (SWPBS) to enhance outcomes for students and staff. Consequently, Illinois established layered support structures to facilitate scaling up SWPBS. This paper describes the development of this infrastructure and presents the results of HLM analyses exploring the effects of implementing SWPBS, with and without fidelity across time, on student behavior and academic outcomes (office discipline referrals, suspensions, and state-wide test scores in reading and math) for a sample of 428 Illinois schools implementing SWPBS. Results indicate that (a) most schools implemented with fidelity and maintained or improved student performance across time and (b) implementation fidelity was associated with improved social outcomes and academic outcomes in math. Study limitations and implications are discussed. (Contains 2 tables, 1 figure, and 2 notes.)