In this review, we bring our personal experiences to showcase insulin from its breakthrough discovery as a life-saving drug 100 years ago to its uncovering as the autoantigen and potential cause of type 1 diabetes and eventually as an opportunity to prevent autoimmune diabetes. The work covers the birth of insulin to treat patients, which is now 100 years ago; the development of human insulin, insulin analogs, devices, and the way into automated insulin delivery; the realization that insulin is the primary autoimmune target of type 1 diabetes in children; novel approaches of immunotherapy using insulin for immune tolerance induction; the possible limitations of insulin immunotherapy; and an outlook on how modern vaccines could remove the need for another 100 years of insulin therapy.