Type 1 diabetes is a chronic autoimmune disease resulting from T cell-mediated destruction of insulin-producing beta cells within pancreatic islets. Disease incidence has increased significantly in the last two decades, especially in young children. Type 1 diabetes is now predictable in humans with the measurement of serum islet autoantibodies directed against insulin and beta cell proteins. Knowledge regarding the presentation of insulin and islet antigens to T cells has increased dramatically over the last several years. Here, we review the trimolecular complex in diabetes, which consists of a major histocompatibility molecule,self-peptide, and T cell receptor, with a focus on insulin peptide presentation to T cells. With this increased understanding of how antigens are presented to T cells comes the hope for improved therapies for type 1 diabetes prevention.