The novel protein kinase C (PKC) isoform, PKC theta, is selectively expressed in T lymphocytes and is a sine qua non for T cell antigen receptor (TCR)-triggered activation of mature T cells. Productive engagement of T cells by antigen-presenting cells (APCs) results in recruitment of PKC theta to the T cell-APC contact area--the immunological synapse--where it interacts with several signaling molecules to induce activation signals essential for productive T cell activation and IL-2 production. The transcription factors NF-kappa B and AP-1 are the primary physiological targets of PKC theta, and efficient activation of these transcription factors by PKC theta requires integration of TCR and CD28 costimulatory signals. PKC theta cooperates with the protein Ser/Thr phosphatase, calcineurin, in transducing signals leading to activation of JNK, NFAT, and the IL-2 gene. PKC theta also promotes T cell cycle progression and regulates programmed T cell death. The exact mode of regulation and immediate downstream substrates of PKC theta are still largely unknown. Identification of these molecules and determination of their mode of operation with respect to the function of PKC theta will provide essential information on the mechanism of T cell activation. The selective expression of PKC theta in T cells and its essential role in mature T cell activation establish it as an attractive drug target for immunosuppression in transplantation and autoimmune diseases.