Regulatory T cells (Tregs) are a suppressive subset of CD4+ T helper (Th) cells important for the regulation of immune responses. Tregs are defined by expression of the transcription factor Foxp3. Additional Treg markers include constitutive expression of the high-affinity IL-2Rα chain (CD25) and cytotoxic T lymphocyte-associated antigen 4 (CTLA-4), along with low expression of the IL-7Rα chain (CD127). CD4+CD25+ Foxp3+ Tregs can be divided into 2 main types: thymically derived Tregs (tTregs) and peripherally derived Tregs (pTregs). Although it is difficult to distinguish between tTregs and pTregs phenotypically, both are thought to have an essential role in immune regulation. Because of their immunoregulatory function, Tregs are an attractive therapeutic target in many different immune-mediated diseases, including transplantation, autoimmunity, and autoinflammation.