Peripheral self-tolerance and immune homeostasis are maintained, at least in part, by the balance between Treg and effector T cells. Naturally, arising CD25(+)CD4(+) Treg, which express the transcription factor Foxp3, suppress the activation and proliferation of other lymphocytes in multiple ways. A CTLA-4-dependent suppressive mechanism is shared by every Foxp3(+) Treg at any location and its disruption breaches self-tolerance and immune homeostasis, suggesting that it is a core mechanism of suppression. Depending on the environment, Foxp3(+) Treg also differentiate to exhibit additional suppressive mechanisms, including the secretion of immunosuppressive cytokines. Naïve T cells acquire Foxp3 expression and suppressive activity under certain in vivo and in vitro conditions, whereas some Foxp3(+) T cells may lose Foxp3 and suppressive activity following proliferation in an IL-2-deficient environment. Moreover, activated effector T cells frequently secrete suppressive cytokines, such as IL-10, in a negative feedback fashion. These findings, when taken together, indicate that peripheral immune tolerance and homeostasis are dynamically maintained by functional differentiation within the Foxp3(+) population, occasional conversion between Treg and non-Treg cells, and the interactions among them. These dynamics provide ample opportunities for immune intervention for the benefit of the host.