Type 1 diabetes mellitus results from an autoimmune disease which specifically destroys the pancreatic beta cells. The disease presents a multigenic susceptibility and the activation of autoreactive T lymphocytes plays a major role in its pathogenesis. Environmental factors may initiate or modulate the immune response. Several mechanisms are probably involved in the ultimate beta cell destruction. The antigens which are first recognized by the immune system are still unknown. Humoral and cellular specific markers of the disease can be detected long before the onset of diabetes. Understanding the mechanisms which trigger the disease should allow a treatment of the autoimmune disease before the clinical expression of diabetes.