The etiological heterogeneity of idiopathic diabetes has been recognized for 25 years, and subdivision into type 1 and type 2 diabetes is fundamental to the way we think about the disease. Review of the literature suggests that the concept of type 1 diabetes as an immunemediated disease emerged rapidly over the period from 1974 to 1976 and showed many of the features of a classic paradigm shift. A few key observations triggered recognition and acceptance of the new paradigm, but the necessary context was provided by scientific developments in areas mainly unrelated to diabetes. The disease paradigm established by 1976 is still widely accepted, and its essential features have been modified only in detail by the revolution in molecular biology that has occurred over the intervening period. Notwithstanding, some of the underlying assumptions remain imprecise, unchallenged, or unconfirmed. Appreciation of the historical origin and subsequent evolution of these fundamental concepts could stimulate critical analysis and help prepare the way for a new paradigm.