Regulatory T cells (T(reg) cells) characterized by expression of the transcription factor Foxp3 play a key role in immune homeostasis. Rather than a monomorphic population strictly determined by Foxp3 as a 'master regulator', the emerging view is one of T(reg) cells as a population with many levels of complexity. Several regulatory factors partake in the control of their transcriptional 'signature', with Foxp3 being a key regulator but insufficient and unnecessary to specify all aspects of the lineage. Distinct subphenotypes of Foxp3+ T(reg) cells are found in different anatomical locations. Some subphenotypes specifically control different facets of effector T cell function and, perhaps surprisingly, share transcriptional control elements with the very cells they regulate. This review will focus on these novel aspects of T(reg) cell diversity.