It is now well established that regulatory T (T_R) cells can inhibit harmful immunopathological responses directed against self or foreign antigens. However, many key aspects of T_R cell biology remain unresolved, especially with regard to their antigen specificities and the cellular and molecular pathways involved in their development and mechanisms of action. We will review here recent findings in these areas, outline a model for how T_R cells may inhibit the development of immune pathology and discuss potential therapeutic benefits that may arise from the manipulation of T_R cell function.