Education researchers are being asked to conduct rigorous, scientifically based studies of K–12 curriculum interventions; therefore, the need for measuring fidelity of implementation and empirically relating it to outcomes (the chief rationale for this review) is warranted to ensure internal and external validity. The results of this review indicate that there are too few studies to guide researchers on how fidelity of implementation to core curriculum interventions can be measured and related to outcomes, particularly within efficacy and effectiveness studies, where the requirements for fidelity measures differ. This review attempts to clarify the definition, conceptualization, and measurement of fidelity of implementation and to guide future researchers in understanding how fidelity of implementation can be used to adjust or interpret outcome measures.