The past years have witnessed significant advance in our understanding of critical roles of T cell co-signals in B7-CD28 family molecules in regulating T cell activation and tolerance. New co-signaling molecules have been identified and their functions have been delineated. These co-signaling pathways play overlapping and distinct regulatory roles at various stages of a T cell response. By expressing in appropriate time and location, these pathways have different regulatory functions through independent receptors or on different subsets of lymphocytes. Precise understanding of these pathways will allow the development of novel approaches to treatment of human diseases such as cancer, viral infection, autoimmune diseases and transplantation rejection.Cellular & Molecular Immunology. 2004;1(1):37-42.