Challenges in the measurement of racial disparities in school discipline are a significant barrier to identifying policy and programmatic reforms that are effective at closing gaps. This article reviews key measurement issues and presents a set of empirical analyses as an illustrative case study. Specifically, we reframe the interpretation of discipline data in light of initiatives designed to reduce racial discipline disparities. We also characterize common metrics and recognize several additional ones for use in discipline disproportionality outcome evaluations. Leveraging a statewide policy reform as an example, we report findings from a quasi-experimental evaluation, which demonstrated that the various metrics can point to differing conclusions. We conclude with proposed guiding principles for the selection and use of discipline disproportionality metrics in evaluations.