Type 1 diabetes is an incurable disease associated with risk of serious acute and chronic complications. It is caused by a marked loss in insulin-producing beta cells. Immune intervention at clinical onset can transiently suppress a further decline in residual functional beta cell mass, particularly in young patients with a higher residual insulin producing capacity at start of treatment. It might achieve a higher effect during the preclinical phase when the beta cell mass is not yet severely affected. Such prevention trials can be prepared by identifying individuals at very high risk to develop diabetes within 3 years and presenting signs of declining, yet relatively preserved, functional beta cell mass. This article reviews biomarker screening strategies to select these participants and discusses developments that can facilitate rapid and straightforward conclusions from novel clinical studies.