Fidelity may be defined as the extent to which delivery of an intervention adheres to the protocol or program model originally developed. Fidelity measurement has increasing significance for evaluation, treatment effectiveness research, and service administration. Yet few published studies using fidelity criteria provide details on the construction of a valid fidelity index. The purpose of this review article is to outline steps in the development, measurement, and validation of fidelity criteria, providing examples from health and education literatures. We further identify important issues in conducting each step. Finally, we raise questions about the dynamic nature of fidelity criteria, appropriate validation and statistical analysis methods, the inclusion of structure and process criteria in fidelity assessment, and the role of program theory in deciding on the balance between adaptation versus exact replication of model programs. Further attention to the use and refinement of fidelity criteria is important to evaluation practice.