The development and function of T lymphocytes are regulated tightly by signal transduction pathways that include specific cell-surface receptors, intracellular signaling molecules, and nuclear transcription factors. Since 1988, several families of functionally important T cell transcription factors have been identified. These include the Ikaros, LKLF, and GATA3 zinc-finger proteins; the Ets, CREB/ATF, and NF-kappa B/Rel/NFAT transcription factors; the Stat proteins; and HMG box transcription factors such as LEF1, TCF1, and Sox4. In this review, we summarize our current understanding of the transcriptional regulation of T cell development and function with particular emphasis on the results of recent gene targeting and transgenic experiments. In addition to increasing our understanding of the molecular pathways that regulate T cell development and function, these results have suggested novel targets for genetic and pharmacological manipulation of T cell immunity.