In the past few years it has been become increasingly clear that T cells capable of actively suppressing immune responses are thought to be in part responsible for the maintenance of peripheral self tolerance. In healthy rodents and humans, CD4(+) T cells constitutively expressing the interleukin (IL)-2 receptor alpha-chain (CD25) are able to exert such suppressive function in vitro and in vivo. Despite great efforts in our understanding of the biology of such immunoregulatory T cells, there are still certain points incompletely understood. Although some authors suggest that immunoregulatory cytokines such as IL-10 or transforming growth factor-beta are critical for the suppressive effect of these cells, this is controversial and the exact molecular nature and the targets of suppression are largely unknown. Thus far, until regulatory T cells can be used for diagnostic or therapeutic purposes many questions have to be answered. In this review we summarize the current knowledge on the function and properties of this T cell subset and discuss their potential role in human autoimmune or chronic inflammatory diseases.