Over two decades ago, CD4+ T cells were classified into various T cell subsets. Each subset is characterised by its specific cytokine pattern and effector functions in the immune response. This classification has long been confined to two subsets of helper T cells called Th1 and Th2 cells. Recently, new CD4+ T cell populations have been describe, including natural and inducible regulatory T cells and the proinflammatory Th17 cells. The discovery of these new populations provided a better understanding of the pathophysiology of various diseases, including chronic inflammatory disorders including some of autoimmune diseases, cancers and chronic infections. In this review, the generation of regulatory T cells and Th17 cells, the mechanisms used by these T cells to maintain the physiological balance between inflammatory and immunosuppressive immune responses, and their implication in physiopathology of various diseases will be discussed. © 2010 - Elsevier Masson SAS - Tous droits réservés.