Treatment integrity (also referred to as "treatment fidelity," "intervention integrity," and "procedural reliability") is an important methodological concerning both research and practice because treatment integrity data are essential to making valid conclusions regarding treatment outcomes. Despite its relationship to validity, treatment integrity has been largely overlooked in education research and related fields. Influences over the past 5 years have increased attention to this construct, yet many questions remain in regard to how it should be addressed. The purpose of this article is twofold. first, we provide a brief overview of the current state of (a) conceptual models of treatment integrity; (b) research on assessment, promotion, and relationship to student outcomes; and (c) treatment integrity related to response to intervention models of service delivery. Second, we suggest future directions for research, practice, and policy related to treatment integrity as we move toward a better scientific understanding of this construct. The article concludes with a description of the purposes of this special series. (Contains 2 figures.)