This article describes the development of the Multi-Tiered System of Supports-Interventionist Beliefs Scale (MTSS-IBS). The introduction reviews the importance between practitioner beliefs and MTSS success. Because there is no measure to assess the MTSS beliefs of school interventionists for the success of all students, we used a principal component analysis and a parallel analysis on MTSS-IBS responses from 379 school counselors who worked as MTSS interventionists. These analyses indicated a four-component structure, and subsequent internal consistency and construct validity analyses provided initial evidence of MTSS-IBS psychometric adequacy. We discuss applications of the MTSS-IBS in research and practice for enhancing MTSS implementation.