The purpose of this article is to illustrate the use of process evaluation for understanding study outcomes. Data from the Child and Adolescent Trial for Cardiovascular Health (CATCH), a large school-based field trial, are used. Teacher characteristics, measures of classroom curriculum implementation, and competing influences are linked to changes in dietary knowledge, intentions, and self-efficacy of students in the intervention schools. Multiple regression analyses indicate that teacher characteristics did not predict program implementation. Teacher characteristics and program fidelity, or the number of modifications made to the classroom curriculum during implementation, had direct and independent effects on student outcomes.