Implementation fidelity (dosage, adherence, quality of delivery, responsiveness, program differentiation) is the degree to which a program was carried out to its original intent. The following three studies examined the implementation fidelity of a school-based health promotion social and character development (SACD) program. The Positive Action (PA) program was a 5yr effectiveness trial conducted in 20 elementary (10 control, 10 intervention) schools in Hawai'i from 2001–2006. Study 1 examined teacher- and school-level factors that influenced fidelity of program implementation using a Diffusion of Innovations framework. Implementation was defined as “the amount of the programs' curriculum delivered, and use of program-specific materials in the classroom and in relation to school-wide activities.” Teachers' completed year-end process-evaluation reports for year 2 (N = 171) and 3 (N = 191). Classroom and school-wide material usage were influenced by the amount of the curriculum delivered, which were influenced by teachers' attitudes toward the program and teachers' beliefs about SACD. These, in turn, were shaped by teachers' perceptions of school climate. Study 2 examined the impact of student responsiveness ratings on prosocial behaviors and feelings, and substance abuse rates. Elementary students (N = 2,926) completed 4 year-end surveys assessing prosocial behaviors and feelings (yr1–4), responsiveness towards PA (yr3 and 4), and substance abuse (yr4, n = 458). Longitudinal path models indicated responsiveness was related to positive behaviors and feelings, and reduced substance abuse rates at year 4. Study 3 examined the extent to which control schools self-initiated or received programming of similar content to the intervention being evaluated (program differentiation). One school leader from each school (N = 18), responded to a questionnaire regarding information about the number of SACD programs conducted over the prior 3 academic years. Control schools reported an average of 8.0±5.1 SACD programs vs. 3.6±3.6 (in addition to PA) reported by intervention schools. In conclusion, program developers need to consider: (a) the importance of a supportive school climate on implementation fidelity; (b) why students may or may not like a given program; and (c) the self-initiated programming occurring in control schools. Greater attention to these will assist in the interpretation of positive and negative outcomes from school-based prevention programs.