The autoimmune process focused on B-cells of the islets of Langerhans and leading gradually to their destruction and the development of type I diabetes takes place under the morphological picture of insulitis. The disease affects genetically predisposed individuals. At the end of the eighties knowledge was greatly expanded due to methods of molecular genetics. The majority of authors assumes that in the pathogenesis of insulitis also environmental factors participate. More detailed information is provided on chemical compounds, nutritive substances and viruses.