This paper reviews worldwide efforts to interdict the type 1 diabetes (T1D) disease process, during the stage of evolution of the disease prior to the time of disease onset. The goal of intervention before disease onset is to arrest immune destruction and thus prevent or delay clinical disease. In this regard, there have been several large-scale multicenter randomized controlled clinical trials designed to prevent T1D. These have tested nicotinamide, parenteral insulin, oral insulin, nasal insulin, and the elimination of cow's milk from infant feeding.