Regulatory T cells (Treg) are critical for regulation of tolerance, control immune responses to self-antigens thereby preventing autoimmunity, and limiting responses to foreign antigens thereby minimizing T cell-mediated immunopathology. Recent data indicate that suppression of organ-specific autoimmunity is dependent on the antigen specificity of Treg. An emerging model of Treg action is that organ-specific Treg acquire suppressive activity through activation by dendritic cells expressing specific antigens. Thus, the efficacy of Treg-based therapy should be increased by using antigen-specific Treg rather than polyclonal Treg. It is necessary to identify relevant antigens and to expand antigen-specific Treg from polyclonal populations. Here, we discuss recent techniques for expansion of antigen-specific Treg, function and antigen specificity of Treg and the therapeutic potential of Treg in controlling autoimmune disease and inducing transplant tolerance.