Legislation such as No Child Left Behind (NCLB), the Individuals with Disabilities Education Act, and its reauthorization, the Individuals with Disabilities Education Improvement Act, mandate that evidence-based practices be used in schools. In fact, NCLB requires that "scientifically-based research" be the basis for classroom practices (Smith, Daunic, & Taylor, 2007, p. 121). In addition, with the introduction of response to intervention, schools are required to provide students with effective educational interventions. However, even evidence-based interventions can work only when implemented correctly. The extent to which an intervention is implemented as it is designed is called "treatment integrity", and researchers and practitioners are increasingly called upon to measure treatment integrity when evaluating the effects of their interventions. The purpose of this article is (a) to define treatment integrity and describe its relationship to student outcomes, (b) to describe how treatment integrity data can be collected, and (c) to discuss the important ways that treatment integrity data should be used for decision making. (Contains 1 figure and 1 table.)