IL-2 was originally identified as a growth factor critical for T cell proliferation in vitro. Although the early studies of IL-2 strongly implied an obligate role of IL-2 in T cell growth, it was later shown that mice deficient in IL-2 or in IL-2R developed an unexpected lymphocytic hyperproliferation and subsequent autoimmune disease. In separate studies of autoimmunity, it was observed that a population of CD4⁺ T cells suppressed the induction of autoimmunity in several in vivo models of autoimmune disease. It was not until the characterization of this subpopulation of CD4⁺ T cells demonstrated that they co-expressed the IL-2R-alpha chain (CD25) that the puzzling phenotype observed in IL-2 deficient mice began to be truly explained. The constitutive expression of the IL-2R-alpha chain on CD4⁺CD25⁺ T cells led to the obvious speculation that IL-2 signaling in CD4⁺CD25⁺ T cells was important to these cells. Recent studies have examined the role of IL-2 in the generation, the expansion, the survival and the effector function of CD4⁺CD25⁺ T cells. It is now evident that IL-2 is critical for the development of CD4⁺CD25⁺ T cells and the phenotype observed in IL-2 and IL-2R deficient mice is most readily explained by the absence of these potent suppressors.