Regulatory T cells (Treg) are involved in the maintenance of peripheral tolerance and in the prevention of autoimmune diseases. They are also essential to the regulation of immune responses observed in allergy, transplantation, cancers and infectious diseases. Most of Treg, called natural, differentiate in the thymus and are defined by their expression of CD4, CD25 and the transcription factor FOXP3. However, another population of Treg induced in the periphery has been recently characterized and appears to play an important role as well. In this review, we will discuss certain aspects of Treg, concerning notably their characterization, their role in normal and disease states, their mechanisms of action. Finally, we will discuss the significance of induced Treg.