Three of the four largest and, arguably, most important long-term clinical trials focusing on diabetes in the past 2 decades have had medical nutrition therapy (MNT) as a major element of their experimental therapies: the Diabetes Control and Complications Trial and its follow-up study the Epidemiology of Diabetes Interventions and Complications Study, the Diabetes Prevention Program, and the Look AHEAD (Action for Health in Diabetes) study. The important features of these studies, their major results, and implications for current and future practice of MNT in the prevention and treatment of diabetes are delineated. The evidence suggests that dietetics professionals have an increasingly important role in the early initiation of MNT in the treatment and prevention of diabetes and in establishing the long-term relationships required to shape behavior and sustain the lifestyle habits that translate into significant reductions in the incidence of diabetes-related health outcomes and improved cost-effectiveness over time.