Process evaluation is an essential element of a multicenter trial in order to explain study outcomes and monitor intervention progress. This paper presents the process evaluation model for the Eat Smart School Nutrition Program, the food service intervention component of the Child and Adolescent Trial for Cardiovascular Health (CATCH), and describes the process evaluation strategies developed to assess this program. The paper details (1) measures of program implementation including training, support visits, and program promotion, (2) food service staff characteristics, and (3) measures of mediating variables including secular trends and school meal participation.