A quantifiable measure of teachers' intervention fidelity when delivering curriculum-based interventions allows researchers to interpret the effectiveness of the curricula under scrutiny. Fidelity measures, however, must accurately represent the critical components of an intervention to confirm that the intervention was delivered as intended and to provide evidence that the delivered intervention is the source of change in targeted outcomes. Research Findings: Sixteen fidelity measures used in the Preschool Curriculum Evaluation Research initiative were evaluated on how well they represented teachers' adherence, the quality of their delivery, the extent to which they exposed children to the curriculum, and participant responsiveness. Most fidelity measures insufficiently represented adherence, quality, exposure, and responsiveness. Practice or Policy: Implications focus on the importance of measure development and the need for developers and researchers to define critical components of a curriculum when constructing corresponding measures of fidelity. (author abstract)