Scientific interest in regulatory T cells has revived during the last decade. Initially described in the early seventies as suppressor T cells, the concept of suppressor/regulatory T cells went through turbulent times during the eighties when molecular analysis failed to identify putative suppressor genes. The constructive and elegant cellular experiments on regulatory T cells during the nineties, initiated by Shimon Sakaguchi and co-workers, however have brought these cells back into the limelight. Nowadays, regulatory T cells are regarded as essential components of the immune system, and several different subsets of regulatory T cells have been described. Considerable regulatory function has been attributed to the CD4+CD25+ T cell subset. These cells act by suppressing adaptive and possibly also innate immune responses thereby maintaining or restoring the balance between immunity and tolerance. The suppressive effects of CD4+CD25+ regulatory T cells are cell-contact dependent but a role for soluble factors, particularly in vivo, has been suggested as well. The aim of this book is to bring together recent developments and viewpoints in the field of CD4+CD25+ regulatory T cells and to discuss the potential use of regulatory T cells in immunotherapy of inflammatory diseases. By linking data on regulatory T cells from experimental models with recent findings from the clinic, this topical book will be of interest to immunologists and other biomedical researchers as well as clinicians that are interested in regulation and manipulation of the immune response during (chronic) inflammatory disease.