Regulatory T cells seem to represent the resurrection of the old suppressor T cells. Although of a different phenotype, regulatory T cells are able to suppress many T cellmediated immune responses. While most basic knowledge about these cells is derived from animal studies, the recent identification of these cells in humans has further attributed to their characterisation by <i>in vitro</i> analysis.
Results obtained have led to broad speculations about
therapeutic potential by interference with these regulatory T cells. This review is an introduction to the world of regulatory T cells and contains an historical overview with respect to the identification and characterisation of these T cells. A distinction is made between naturally occurring regulatory T cells (nT<sub>reg</sub>), which require cell-cell contact for suppression, and inducible regulatory T cells (iT<sub>reg</sub>), which predominantly mediate suppression via cytokinedependent pathways. Although only limited studies on regulatory T cells in human disease are available today, the possible clinical applications are discussed in light of the other side of the coin, i.e. the danger of interfering with homeostatic mechanisms in the immune system.