The immune system maintains a delicate balance between self-tolerance and protective immunity, but in transplantation this equilibrium shifts toward alloimmune activation, making regulatory T-cell function and costimulatory control central to achieving durable immunotolerance. The CD28/CD80–86 pathway is a dominant driver of T-cell activation and also a critical regulator of Treg development, survival, and suppressive function, positioning its therapeutic modulation as a compelling strategy to promote graft acceptance. Clinical experience with belatacept demonstrates superior long-term renal function compared with calcineurin inhibitors, although early rejection in select populations highlights the redundancy of costimulatory networks and the need for rational combination approaches. Next-generation agents—including selective CD28 antagonists, engineered biologics, and localized delivery platforms—further expand the therapeutic landscape, while biomarker-guided dosing and integration with regulatory T- and B-cell–based therapies offer opportunities for personalized tolerance induction. This review synthesizes mechanistic insights and clinical advances in CD28-directed costimulatory therapeutics and outlines how emerging agents, precision biomarkers, and regulatory-cell–centered strategies may be leveraged to achieve durable, organ-specific graft acceptance.