Treatment integrity, in its broadest sense, is the extent to which an intervention is implemented as intended. Over the past 3 decades, understanding of treatment integrity has greatly evolved. This book summarizes the current state of our knowledge of treatment integrity on such topics as: (1) how it fits into the larger contexts of evidence-based practice and implementation science; (2) its relation to ethics and the law; (3) conceptual models, assessment, promotion, and application; (4) conceptual models of treatment integrity in the fields of medicine, prevention science, and health psychology; (5) assessment of treatment integrity itself; (6) how a conceptual model can inform the development of a comprehensive treatment integrity assessment system; and (7) its use in practice and research in education and psychology. Authoritative and up to date, this volume is a unique resource for all professionals supervising, providing, or evaluating intervention services, including researchers and practitioners in clinical, counseling, and school psychology; child and adolescent psychiatry; social work; communication disorders; special and general education; program evaluation; and educational leadership.