In 2021, the Council for Higher Education Accreditation (CHEA) made a monumental move to require postsecondary institutions to evaluate and document their actions to ensure fairness in admissions, an inclusive learning environment, and equitable student outcomes. Around the same time, a team comprising educational measurement experts, diversity officers, and accreditation specialists from Brooklyn College of the City University of New York and the University of Florida developed an instrument for assessment of institutional diversity, equity, and inclusion (DEI) called the Diversity, Equity, and Inclusion Institutionalization (DEII) assessment. The goal was to measure campus DEI and improve differential outcomes for its underserved populations. The team formed an advisory board represented by scholars from Harvard University, the Hispanic Association of Colleges and Universities (HACU), and the American Association of Learning in Higher Education (AALHE) to review and provide feedback on the assessment of DEI institutionalization. This article details the process of understanding existing assessment efforts, provides evidence for the rigor of the DEII assessment tool, and shares results from the initial round of data collection. The authors also invite interested institutions to participate in their ongoing process of refining their assessment approach in order to help leaders understand their DEI work at the institution level.