Evidence-based practice (EBP) has become a dominant paradigm in North American behavioral health and social service provision. Once a model of expert decision-making that asked practitioners to search through the best available evidence to inform their clinical decisions and select interventions, EBP is now better understood as a complex system of legitimation that designates particular methods and-by extension-their practitioners as evidence-based. While critics worry that EBP forecloses professional discretion by imposing particular epistemic virtues of intervention science, this ethnographic case demonstrates that 1) EBP legitimates profes-sional actors, methods, and organizations at least as much as it hampers them and 2) a wide range of extra -scientific actors are involved in producing and legitimating the evidence of evidence-based practice, including policy makers, public and private insurers, state agencies, charitable foundations, registries and clearinghouses, health and human service organizations, and helping professionals themselves. Once we recognize the range of actors and institutions involved in basing and legitimating evidence, and the rhetorical work of tethering sci-entific terms to resonant political and economic discourses, we learn that there is nothing self-evident about evidence-based practice. Drawing on the social scientific study of expertise and focusing empirically on how one behavioral intervention earns and retains its status as an EBP, this study traces the trans-institutional life of evidence and the continual need to legitimate it as a base for behavioral health practice.