Theory is a useful guide for program design and implementation. This paper uses a large-scale school-based delinquency prevention project to illustrate the use of theory in program design and the need for line staff to have a clear understanding of the program's theoretical underpinnings. Evaluation results for the program show that the program was effective for reducing the level of delinquent behavior and changing a number of theoretical precursors of delinquent behavior at the school level. The program, however, failed to bring about the desired changes in behavior and attitudes of individuals targeted for special services. The differential effectiveness of the program components is attributed in part to differences in the extent to which implementation was guided by theory. (Author)