In this book on diabetes mellitus both the pathogenesis and treat- ment of the disease will be discussed. Pathogenetic studies have led to the distinction between type I and type II diabetes. In type I hyperglycaemia is due predominantly to insulin deficiency; in type II insulin resistance is more important. Three pathogenetic factors are thought to play a role in the etiol- ogy of type I diabetes: heredity, viral infections and immunity. There may be a relationship between these three aspects since genetic pre- disposition changes the susceptibility to viral infections and/or the immunological defence against these infections. Insulitis ensues. Auto- immune processes initiated by this chain of events may contribute to the destruction of the B cells in the islets of Langerhans and diabetes -may eventually follow. This means that at the time of the sudden out- break of the disease, the destructive process has already been active for years.