The primary function of regulatory T cells, also known as suppressor T cells, is to maintain immune homeostasis. This involves suppression of successful immune responses and control of self versus non-self recognition. Failure of the latter results in autoimmune destruction of host cells and tissue. Like other T cells, regulatory T cells mature in the thymus where they are characterized by the variable expression of CD8, CD4, CD25 and FoxP3. The importance of FoxP3 is underlined by genetic mutations in this molecule which result in a fatal autoimmune disorder known as Immune dysregulation, Polyendocrinopathy, Enteropathy X-linked (IPEX) syndrome. Conversely, taking advantage of the immunosuppressive potential of regulatory T cells is an important area for advancement in the fields of autoimmune disease and organ transplantation.