In summary, many new modalities of immunosuppression after transplantation are being investigated (Fig. 1). These approaches include various new drugs or monoclonal antibodies that target different cell subsets, cellular activation pathways, cellular effector function or mediators (such as cytokines) of effector function, ligands that stabilize cellular interactions, or antimetabolites that preferentially affect lymphocytes (Tables 4 and 5). Because of the excellent early graft and patient survival results after liver transplantation under various current immunosuppressive protocols, future clinical trials using these various new modalities will require large numbers of patients to show statistically significant differences in graft or patient survival. Therefore, other criteria in addition to graft and patient survival must be analyzed to evaluate the importance of new immunosuppressive therapies. These criteria may include incidence of acute or chronic rejection, long-term graft function, incidence of infectious complications, length of hospitalization, drug toxicity, and patient tolerance and compliance with new therapies.