Various environmental triggers, e.g. certain viruses and dietary factors, are thought to initiate the autoimmune process, leading to the destruction of pancreatic beta-cells and consequent Type 1 diabetes. A genetic predisposition is another prerequisite allowing the autoimmune process to progress. Twin studies, major geographical variations in incidence rates, temporal trends in the incidence and findings in migrant studies indicate that environmental factors play a crucial role in the development of Type 1 diabetes. In the present review the major focus is on dietary factors, and among them particularly the possible role of cow's milk proteins. The cow's milk and Type 1 diabetes hypothesis was developed more than 10 years ago, and the issue is still not settled. Among viral infections, enteroviruses are today the most interesting group of viruses in this respect, as recent prospective studies indicate that these viruses may trigger and potentiate existing beta-cell autoimmunity. Among toxins, particularly N-nitroso compounds are of potential interest, as they are probably involved in the aetiology of some cases. Finally, psychosocial factors and the interaction between genetic predisposition and environmental factors are briefly discussed.