T regulatory cells, also known as Tregs, are essential in the maintenance of immune homeostasis and self-tolerance. Naturally occuring Tregs (nTregs) are characterized by the expression of CD4, CD25 and FOXP3, which is a transcription factor important in the development of Tregs. In addition to nTregs, there are several distinct subsets of induced regulatory T cells (iTregs), including Tr1 and Th3. Tregs limit immune activation through a variety of direct and indirect interactions, many of which remain to be defined. Fully understanding Tregs will lead us to harnessing the capacity of these cells in order to develop strategies to limit and prevent autoimmune disorders, tolerance to transplantation, and potentially boosting immune activity against cancer cells.