This study focused on the development of the Multi-Tiered Systems of Support--Sustainability Scale (MTSS-SS). Review of the literature identified factors associated with sustainability for multi-tiered systems of support (MTSS), and it indicated few sustainability measures currently exist for practitioners and researchers to incorporate into MTSS program evaluation. This study endeavored to create a brief measure of MTSS sustainability that could be used for evaluation in research and practice by school interventionists. Study participants included a national sample of 598 school counselors and school psychologists who worked as interventionists in schools using MTSS. This group completed the 10-item MTSS-SS. Outcomes from content validity ratings, principal axis factoring, internal consistency reliability analyses, and construct validity with known groups indicated the MTSS-SS possessed initial evidence of psychometric adequacy when used by interventionists from the disciplines of school counseling or school psychology. Discussion focuses on the use of the MTSS-SS, need for further development, and study limitations.