Fidelity of implementation is an essential factor in realizing outcomes of evidence-based interventions. Yet perspectives vary on the degree to which fidelity is necessary or desirable, implementers may receive limited guidance on fidelity, and research on implementers' understanding and operationalization of fidelity is lacking. We conducted key informant interviews with 20 individuals who implemented an evidence-based teen dating violence prevention curriculum in seven sites. Interviews addressed how implementers defined fidelity and the adaptations they made in implementing the curriculum. Although all implementers reported delivering the curriculum with fidelity, their definitions of fidelity varied. Most defined fidelity in terms of adherence to the published curriculum, although definitions sometimes allowed modifications of pedagogy or adding or subtracting materials. A smaller group of implementers defined fidelity in terms of their perceptions of the curriculum's core messages. All implementers described variations from the published curriculum, which were frequently inconsistent with their own definitions of fidelity. Implementers committed to the value of implementation with fidelity may need support in its definition and application. Developers can support implementers by communicating program theory in terms that implementers will understand, clarifying expectations for fidelity and allowable adaptations, and codifying lessons learned from previous implementation.