This multimethod study generated and tested a "best evidence" model of school improvement processes (SIP) capable of improving student achievement. Initially developed through the review of a comprehensive body of previous empirical research, the model was further refined through a 2, 5-year longitudinal study in 10 schools. A quantitative test of this refined model was then conducted using survey evidence from administrators, teachers, parents, and students in 100 elementary schools. The model as a whole explained modest but significant amounts of variation in student achievement across schools. School leadership and SIP implementation processes accounted for the largest proportion of explained variation. (Contains 5 tables, 1 figure, and 3 notes.)