The purpose of this instrumental, multisite case study is to examine fidelity, adaptation, and differentiation challenges found at Wood Badge, a nationwide Boy Scouts of America training for adult volunteer leaders. Our iterative analysis of more than 900 pages of fieldnotes and 400 pages of documents revealed facilitators often explicitly taught syllabus content during the trainings. Observers noted 119 minor differentiations across trainings, notably involving facilitator delivery methods and the duration and scheduling of training segments. Facilitators observed 16 adaptations, which appeared to be based on external conditions at trainings or facilitator preferences, and just three instances of differentiation. Our analysis of the trainings surfaced key fidelity/adaptation issues like overadherence, conflicting notions of deviation, and the impact of preparation on fidelity. In addition, we identified factors influencing facilitators' use of adaptation and differentiation. Recommendations for large-scale trainings are made based on the study's findings.