This volume covers many topics in the field of T‐cell costimulation. The need for such a volume is testament to the growth of the field. From its beginning as a concept in the 1980s, we have now progressed to the point where many molecules now have functionally defined roles in T‐cell costimulation. In addition, the field has progressed ‘from bench to bedside’. Abatacept [cytotoxic T‐lymphocyte antigen‐4 (CTLA‐4)‐immunoglobulin (Ig) (CTLA‐4‐Ig)], an inhibitor of CD28‐mediated T‐cell costimulation, was approved for the treatment of moderate‐to‐severe rheumatoid arthritis in 2006 by the Food and Drug Administration and in 2007 by the European Medicines Agency. This chapter first presents a personal historical perspective on the early basic studies on the elucidation of the CD28/B7 T‐cell costimulatory pathway and the discovery of CTLA‐4‐Ig. We next present an overview of studies of CTLA‐4‐Ig in preclinical animal studies. The material discussed in these first two sections is selective rather than exhaustive; their purpose is to provide context for the final section, a summary of human clinical studies performed with abatacept.