This article draws on previously employed methods for conducting fidelity studies and applies them to an evaluation of an unprescribed intervention. We document the process of assessing the fidelity of implementation of the Math Recovery first-grade tutoring program, an unprescribed, diagnostic intervention. We describe how we drew on recent conceptions and standards of fidelity assessment to evaluate the implementation of the program, detailing the process of developing and testing instruments for assessing implementation fidelity, including (a) identifying the intervention's program theory and core components; (b) creating operational definitions of the intervention's core components; (c) developing coding instruments; (d) selecting and training coders; (e) instituting a sampling frame sufficient for generalizing fidelity findings to the study population; and (f) determining the reliability and validity of the data. We also provide a brief overview of the results of the fidelity assessment. We conclude by discussing the aspects of our work that have implications for assessments of fidelity of implementation of unprescribed interventions more generally.