The purpose of this paper is to describe the process evaluation model for the classroom curricula of the Child and Adolescent Trial for Cardiovascular Health (CATCH) Project. The process evaluation plan specifically targets how much each curriculum was implemented, to what degree it was implemented as designed, and the barriers to implementation. Additionally, the rationale for each of the process evaluation measures and the instrument development process are presented. Data resulting from these measures will be essential in order to answer questions regarding the internal validity of the main outcomes of the project. Specific examples and sample results are provided from the CATCH third-grade classroom curriculum, which was implemented the first year of CATCH. A discussion also is presented of how the findings from a sample of these measures were used to gain additional insight on the salient features of the curriculum, and how those features may be related to student outcomes.