It is not unusual for antigens and potentially responsive T cells to co-exist in the same organism while these T cells remain silent and do not mount life-threatening immune responses. A rich array of mechanisms has been proposed to explain these observations. T cell silencing is controlled in multiple levels. Initially, dendritic cells and regulatory T cells appear to play critical roles. In addition, T cell immunity is tightly regulated by a molecular network of cytokines and cell receptor interactions by the opposed surfaces of antigen-presenting cells and T cells. Recognition of a specific antigen is therefore shaped and tuned by co-stimulatory and co-inhibitory receptor-ligand pairs. At last, immunologists are beginning to exploit the rules governing these assorted sounds of T cell silence.