A life-span developmental approach describes Joan McCord's career and highlights her pioneering contributions to criminology and, more broadly, to understanding human development. The main focus of this article is on her exceptional scientific contributions through the assessment of the Cambridge-Somerville Youth Study experimental preventive intervention. We highlight her efforts to understand how a delinquency prevention intervention caused iatrogenic effects and the lessons she drew for evaluation research. Important contributions to key issues in developmental criminology are summarized, such as the different roles of mothers, fathers, and neighborhoods in the development of delinquency as well as the importance of differentiating discipline from punishment. We describe how Dr. McCord relied on philosophy, how she tackled oppositions between theory-driven and data-driven research in criminology, and how she helped young investigators learn how to learn, and we end by highlighting her contributions to the organization and development of criminology in the United States and around the world.