The purpose of this study was to identify, categorize, and describe practitioners' perspectives regarding factors that help and hinder sustainability of Tier I (universal) systems within School-Wide Positive Behavioral Interventions and Supports (SWPBIS). Seventeen participants involved in sustaining Tier I SWPBIS over several years within a school district were interviewed and asked what events affected its long-term implementation through a qualitative approach called the Critical Incident Technique (CIT). A total of 227 critical incidents were recorded and sorted into emergent unitary clusters based on content analysis. These categories then underwent rigorous reliability and validity checks, including expert analysis, inter-rater agreement, and participant feedback. This process yielded 13 categories that represent the participants' experience of sustainability: Continuous Teaching, Positive Reinforcement, SWPBIS Team Effectiveness, Staff Ownership, School Administrator Involvement, Adaptation, Community of Practice, Use of Data, Involving New Personnel, Access to External Expertise, Maintaining Priority, Staff Turnover, and Conflict of Personal Beliefs/Mistaken Beliefs.