The adoptive transfer of T cells isolated or engineered to have specificity for diseased cells represents an ideal approach for the targeted therapy of human viral and malignant diseases. The therapeutic potential of adoptive T cell therapy for infections and cancer was demonstrated in rodent models long ago, but the task of translating this approach into an effective clinical therapy has not been easy. Carefully designed clinical trials have evaluated the transfer of antigen-specific T cells in humans, and provided insight into the barriers to efficacy and strategies to improve T cell therapy. The importance of altering the host environment to facilitate persistence and function of transferred T cells and intrinsic properties of T cells that are selected or engineered for therapy in determining their fate in vivo are key issues that have recently emerged and are informing the design of the next generation of clinical trials.