Implementation fidelity is often thought of as a necessary condition to achieve internal validity and as having a relation to student outcomes. This relation is increasingly modeled; we reviewed reading intervention studies for students in pre-K-12 in which measures of implementation fidelity were included in final data analysis. A systematic search yielded 50 studies using mostly measures of dosage, adherence, and quality. We provide a detailed, narrative description of the construction of the measures, revealing a wide variety of conceptualizations of implementation fidelity, in terms of content, measurement approaches, and variable construction. The original studies reported varied estimates of the relation between implementation fidelity and student outcomes. To better study the relation in the future, we recommend researchers incorporate measurement of implementation fidelity in the design phase of research projects.