Regulatory T cells (also referred to as suppressor T cells) are important components of the homeostasis of the immune system, as impaired regulatory T cell activity can cause autoimmune diseases and atopy. It is now clear that the phrase ‘regulatory T cells’ encompasses more than one cell type. For instance, CD4⁺CD25⁺ regulatory T cells have received attention due to their immunosuppressive properties in vitro and in vivo, but in several instances it has been shown that CD4⁺CD25⁻ T cell populations also contain potent regulatory activity. Recent progress in the field of regulatory T cells includes the discovery of the role of two tumor necrosis factor receptor (TNFR) family members (GITR and TRANCE-R/RANK) in Treg biology, the improved understanding of the role of co-stimulatory molecules and cytokines IL-10 and IL-2 in the induction and function of Tregs, and the generation of CD25⁺ and CD25⁻ regulatory T cells in vivo through high-avidity T cell receptor interactions.