Sustainability of effective practices in schools is a critical area for research in any domain. The purpose of this article is to describe and evaluate the validity and reliability of a recently developed research instrument designed to evaluate schools' capacity to sustain school-wide positive behavior support (SWPBS) efforts at the universal tier. The "School-Wide Universal Behavior Sustainability Index-School Teams" (SUBSIST) was created to assess factors (of the context, implementer practices, and outcomes) that enhance or prevent sustainability of SWPBS. Content of the web-based survey was identified through literature review, and initial validation analyses included ratings of content validity by an expert panel (n = 21) and assessment of internal consistency, test-retest reliability, interrater reliability, and concurrent validity (with SWPBS fidelity of implementation data) through a pilot study (n = 25). Results indicated strong psychometric properties for assessing sustainability. The authors discuss the results in terms of future research in enhancing SWPBS sustainability. (Contains 2 tables and 2 figures.)