This paper examines measures used by studies associated with Preschool Curriculum Evaluation Research Initiative (PCER) funded by the Institute of Education Sciences. Analysis of 17 measures of fidelity used by 13 curriculum interventions highlights how research in preschools is and is not measuring fidelity of implementation. The following research questions are pursued: (1) In what ways do the measures used in these studies represent the three primary elements of implementation: (a) adherence, (b) exposure, and (c) participant responsiveness?; and (2) Do studies effectively use measures to differentiate between teachers' fidelity to crucial elements of the curriculum and teachers' general instructional quality (i.e., high fidelity as compared to good teaching)? Overall, analysis of these 16 fidelity measures reveals that the corresponding preschool studies insufficiently measured the primary components of fidelity. In many cases, the measures used in these studies were more effective at assessing general quality of instruction and less successful at evaluating fidelity of implementation. Despite increased attention, measurement of implementation fidelity in preschool research continues to be challenging and development of measures that more comprehensively represent implementation fidelity more is essential. (Contains 1 table.)