The face of immune-mediated (type 1) diabetes is changing. No longer considered a disease confined to childhood, the incidence rate in Western countries is clearly rising and affecting younger children. Such a secular trend can only be explained on the basis of increased contacts with adverse environmental factors acting on a background of complex genetics. Multiple defects in immunological tolerance to "self' predispose to immune-mediated (type 1) diabetes. Initiation of immune responses involves the cytokine rich natural killer T cells. Such cells appear deficient in both humans and the rodent models of the disease. Furthermore, the regulatory abilities of T cells in general seem to be compromised. Effector mechanisms probably are dominated by cell-mediated beta cell destruction through apoptosis induction. Surprisingly, the essential antigen-presenting cells in the autoimmune processes involved appear to be B lymphocytes. The improved understanding of the beta cell autoantigens involved has led to better disease prediction. The long prodromal phase now readily identifiable through autoantibodies is spawning hopes of disease prevention, notably through antigen-based interventions or diabetes "vaccines."