Cellular assays have been developed to test for various effector, cytotoxic, and regulatory functions of T cells and have been used throughout the history of clinical transplantation to assess the immune profile of solid organ and marrow recipients. One goal of these cellular studies has been to determine if posttransplant changes in the donor antigen-specific cellular response could predict good and poor graft outcome, thereby allowing for individualization of immunosuppression. This review outlines the use of established and newly developed cellular assays to assess the dynamic processes of the posttransplant immune response and to provide insights into the mechanisms involved and potential points for intervention.