<p>The purpose of this article is to describe the initial development and beta testing of a fidelity measure designed to assess the implementation of elementary school Integrated Multi-tiered Systems of Support. Given the challenges associated with implementing Integrated Multi-tiered Systems of Support and the lack of a psychometrically validated tool for measuring it, there was a need to develop such a tool that could be used by both researchers and practitioners. We present five research-based steps our study team used to develop and test the Integrated Multi-tiered Systems of Support Fidelity Rubric and the supplementary materials associated with it. The steps included reviewing existing fidelity measures, identifying common concepts and scales used across these measures, involving an expert group, conducting practitioner focus groups, and beta testing for measure refinement in preparation for formal psychometric testing. Our article also includes a discussion of the benefits of following a structured approach to develop a fidelity measure. The information in this article is useful to practitioners and researchers seeking to learn about the origins of the measure, as it is now publicly available, used in schools across the country to assess and support implementation fidelity, and included in studies and evaluations of Integrated Multi-tiered Systems of Support. Other fidelity measure developers may also find the article useful in informing their measure development and beta testing process.</p>