Because of its widespread adoption and implementation (in over 13,000 schools in the US; Center on Positive Behavioral Interventions and Supports, 2010), there has been increasing attention to how School-wide Positive Behavior Support (SWPBS) systems can be sustained. Sustained implementation can be defined as "continued use of an intervention or prevention program, with ongoing implementation fidelity to the core program principles, after supplemental resources used to support initial training and implementation are withdrawn" (Han & Weiss, 2005, p. 666). The term sustainability regards a practice's potential for durable implementation with high fidelity, when considering features of the practice, its implementation, and the context of implementation. Though it can be tempting to consider sustainability as deriving entirely from the behavioral principle of maintenance, sustainability includes not only maintenance, but also ongoing adaptations to enhance a practice's effectiveness, efficiency, and contextual fit (Elias, Zins, Graczyk, & Weissburg, 2003; McLaughlin & Mitra, 2001). This process of adaptation is known as continuous regeneration (McIntosh, Horner, & Sugai, 2009). Given the importance of sustainability to continued positive outcomes for stakeholders and wise use of resources, there is an urgent need to explore theories of sustainability and glean practical information to make practices, including SWPBS, more sustainable. [In C. R. Reynolds, K. J. Vannest & E. Fletcher-Janzen (Eds.), "Encyclopedia of special education: A reference for the education of children, adolescents, and adults with disabilities and other exceptional individuals" (4th ed., pp. 2061-2064). Hoboken, NJ: Wiley.]