Autoimmune diseases afflict approximately 5% of the population and reflect a failure in the immune system to discriminate between self and non-self resulting in the breakdown of self-tolerance. Regulatory CD4+CD25+ T cells (Treg cells) have been shown to play an important role in the maintenance of immune homeostasis and self-tolerance by counteracting the development and effector functions of potentially autoreactive T cells. We have in the previous APMIS review described the phenotype and physiology of Treg cells. The present overview deals with the thymic origin of Treg cells and their role in disease models such as autoimmune gastritis and inflammatory bowel disease. Finally, we will consider some aspects of the therapeutic potential of Treg cells.