Regulatory T (Treg) cells, as central mediators of immune suppression, play crucial roles in many facets of immune systems. The transcription factor Foxp3 has been characterized as a master regulator of Tregs, and is induced during their thymic development. Foxp3⁺ Tregs can also be generated from naïve T cells after stimulation in the presence of TGF-β and IL-2; the resulting cells are called induced Tregs (iTregs) when generated in vitro, or peripheral Tregs (pTregs) when generated in vivo. Compared to tTregs, iTregs have been shown to be unstable, and attempts to generate stable iTregs have been made for clinical applications. We review here the current knowledge on the development of pTregs, iTregs, and their roles and applications.