Insulin-dependent diabetes mellitus (IDDM) is the consequence of a chronic process leading to the destruction of insulin producing beta cells of the pancreatic islets. Although the etiology of the disease is as yet unclear, the genetic background, environmental factors and the immune system are known to be the key factors contributing to the induction of the autoimmune process. The precipitating events leading to IDDM are largely unknown. In recent years research on the pathogenesis of IDDM has identified an array of pancreatic islet autoantigens which are recognized by the immune system. The islet cell autoantigens are principally recognized by autoreactive lymphocytes and autoantibodies present in the blood of the patients. Islet specific lymphocytes are thought to directly cause the disease while the antibodies have been used as valuable markers of IDDM and its preclinical phase. In this review we shall focus on cellular and humoral immunity to islet cell antigens which have been suggested to play a major role in the pathogenesis of IDDM and are useful tools for the prediction of the disease.