CD25+CD4+ Regulatory T cells (Treg) represent a unique population of lymphocytes capable of powerfully suppressing immune responses. A large body of experimental data have now confirmed the essential role played by these cells in a host of clinically relevant areas such as self-tolerance, transplantation, allergy and tumor/microbial immunity. Despite this mass of knowledge, significant gaps in our understanding of fundamental Treg biology remain, particularly regarding their development and mechanisms of suppression. In this review we attempt to highlight the current controversies and directions in which this exciting field is moving.