This book addresses one of the major challenges of immunology today that is being directed to the translation of the rapidly emerging volume of basic science contributions of immunology to clinical medicine. In so doing, the book systemically introduces and discusses concepts, classifications, phenotypic and functional descriptions of regulatory T (Treg) cells in health and disease. The authors of the 15 chapters were selected from among the most qualified experts in the field of Treg cell research who provide a comprehensive overview of Treg cells and their biology in the ensuing chapters. The beginning chapters provide a useful contemporary classification of Treg cell populations and then progress to chapters that explore basic mechanisms of Treg cell function and epigenetic control. In addition to descriptions of typical CD4+ Foxp3+ cells, other chapters provide detailed presentations of Treg subsets such as CD8+ Tregs and IL-10-producing Tr1 cells. The differences of various Treg subsets, as well as circulating and resident Treg cell populations, are next compared. Importantly, the next chapters provide the clinical correlation of Treg cells with autoimmune diseases, inflammatory diseases, metabolic diseases, cancer and organ transplantation and progress to chapters that highlight emerging innovative technology including nanoparticle-Treg cells and their translational values. In summary, the book will provide a valuable resource not only for graduate students and researchers in the fields of immunology, cell biology and translational medicine but also for all others interested in learning more about Treg cells and their application in human health and disease.