In the United States, the Child Protective Services system is responsible for investigating and responding to allegations of child abuse and neglect. At the conclusion of an investigation, caseworkers are expected to decide whether allegations are “substantiated” (demonstrated to be true) or not. How that decision is made—and whether it reflects an objective assessment of the evidence available—is widely debated. This chapter first presents an overview of the decision-making process and the implications of decision-making for vulnerable children and families. Next, it describes how rates of substantiation vary across and within states. The authors then present data from a nationally representative study of child protective services investigations on the factors associated with the decision to substantiate child maltreatment. They find that agency characteristics are predictive of substantiation, net of child and family characteristics. Overall, the authors conclude that substantiation is unlikely to be a valid indicator of the incidence of child maltreatment, and they discuss possible strategies for improving the consistency and utility of the substantiation decision.