During follow-up of the suppressive functions of CD4+ T helper (Th) 2 cells in recent years, the suppressive capacities of newly recognised CD4+ Th cells with more widespread suppressive potential have been extensively investigated. These Th cells, collectively termed regulatory T cells, are characterised by the secretion of specific cytokines, such as interleukin (IL)-10 (Tr1 Th cells), transforming growth factor (TGF)beta (Th3 cells) or the constitutive expression of CD25 (naturally occurring T regulatory cells, nTregs). The balance of these regulatory T cells with pro-inflammatory effector T cells, such as Th1 (interferon (IFN)gamma secreting), Th17 (IL-17 secreting) and CD25- Th cells, has been shown to be of pivotal importance for the development and persistence of autoimmune diseases. The high potential of regulatory T cells (in particular nTregs), to efficiently suppress several arthritic responses both in humans and in animal models of arthritis, make them therapeutic targets of interest in arthritic conditions such as rheumatoid arthritis.