The NOD mouse was discovered and established as an inbred strain in Japan. It is an excellent animal model for human Type 1 (insulin-dependent) diabetes mellitus in many aspects, including genetics, immunology, virology, and prevention and therapy. The diabetes and/or insulitis is controlled by at least 10 genes and results from the T cell-mediated destruction of beta cells. Retrovirus might also play a role in the pathogenesis. Insulitis and/or diabetes of the mice is easily prevented by a number of agents or manipulations, suggesting that diabetes of the mice develops only when many diabetogenic factors assemble. Some of the intervention trials using the mice are hoped to be applied to human Type 1 diabetes.