The purpose of this exploratory 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 Critical Incident Technique. Two hundred and twenty-seven 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. [This paper was published in "Journal of Special Education" v49 n3 p157-167 2015.]