The immune system is challenged by randomly generated immune receptors that by chance can recognize self-antigens. Immunological tolerance functions as a fundamental concept in the control of a broad spectrum of immune responses not only to autoantigens but also to foreign antigens. During the past decade, CD4+ CD25+ regulatory T-cells (Tregs) have emerged as key players in the development of immunological tolerance. This review will present an update on the current knowledge about the phenotype, function, and clinical relevance of this regulatory T-cell population. The therapeutical potential of Tregs to specifically suppress immune responses in autoimmunity and transplantation and their inhibitory effects in anti-tumor immune responses will be discussed.