The immune system reacts vigorously to foreign invaders such as viruses, bacteria, protozoa, and parasites, and yet shows unresponsiveness to our own proteins. In the past decade, several types of T lymphocytes in the immune system have been discovered to function as regulatory cells to suppress unwanted immune responses to self-proteins, leading to the protection of autoimmunity. Furthermore, regulatory T cells also protect us from the development of asthma allergy, and prevent organ transplantation rejection. Regulatory T Cells and Clinical Application provides a comprehensive view of all types of regulatory T cells described so far in the literature. By thoroughly reviewing the immunobiology of regulatory T cells and their implications in various forms of human diseases, a new perspective of therapy using regulatory T cells as individualized medicine to treat inflammatory diseases including diabetes, allergy and transplantation rejection is highlighted.