The exquisite sensitivity and specificity of T cells for their targets offers considerable promise for the use of immunotherapy in cancer control. However, in many instances, T cells found within tumors are dysfunctional. A major cause for this dysfunctional state is the expression of inhibitory molecules. We have found that the expression of inhibitory molecules by both murine and human tumor infiltrating CD8+ cytotoxic T cells correlates strongly with the expression of the transcription factor BLIMP-1. In this application we show that tumor infiltrating CD8+ T cells over-express BLIMP-1 compared to CD8+ T cells responding to acute viral infection. We show that limiting BLIMP-1 expression in tumor infiltrating lymphocytes results in reduced inhibitory molecule expression and increased functional activity. Further, we show that BLIMP-1 expression controls CD8+ T cell function beyond the expression of inhibitory molecules. From these data, we propose to pursue studies that will define how BLIMP-1 expression is regulated in tumor infiltrating lymphocytes, and will determine how BLIMP-1 expression regulates tumor infiltrating lymphocyte function. The studies presented here will illuminate the mechanisms behind tumor infiltrating lymphocyte dysfunction, and will provide opportunities to develop interventions that either prevent dysfunction, or restore activity.