T cells are central mediators of adaptive immunity. As such, they are involved in both normal immune responses (e.g., rejection of a transplanted organ) and abnormal ones (e.g., rheumatoid arthritis). T cells require both antigen-specific and costimulatory signals for their full activation. Advances in protein engineering and an increased understanding of the immune response have culminated in the evolution and creation of protein therapeutics that target specific costimulatory molecules. The selective costimulation modulator abatacept (CTLA-4Ig) binds to CD80 and CD86, blocking interaction with CD28, and is approved for the treatment of moderate to severe rheumatoid arthritis. Belatacept, currently enrolling phase III trials in renal transplantation, was rationally designed from abatacept to bind with more avidity to CD86, providing the more potent immunosuppressive properties required for immunosuppression in transplantation. This review describes the relevant immunology and summarizes recent clinical findings on these two molecules. Although both inhibit the CD28 costimulatory pathway, they are tailored for specific disease states--abatacept for autoimmune diseases and belatacept for transplantation.