Genetic contributions to the cause of type 1 diabetes have been studied for more than 30 years, but only recently, with modern genetic tools, has the importance of seemingly minor contributors been appreciated. This article reviews recent advances in knowledge of the genetics of type 1 diabetes and shows how this information could find clinical applications of considerable consequence. This article reviews recent advances in knowledge of the genetics of type 1 diabetes and shows how this information could find clinical applications of considerable consequence. In 1976, the noted human geneticist James Neel titled a book chapter “Diabetes Mellitus: A Geneticist's Nightmare.” 1 Over the past 30 years, however, the phenotypic and genetic heterogeneity of diabetes has been painstakingly teased apart to reveal a family of disorders that are all characterized by the disruption of glucose homeostasis but that have fundamentally different causes. Recently, the availability of detailed information on the structure and variation of the human genome and of new high-throughput techniques for exploiting these data has geneticists dreaming of unraveling the genetic complexity that underlies these disorders. This review focuses on type 1 diabetes . . .