The kinase TAK1 is critical for innate and B cell immunity. The function of TAK1 in T cells is unclear, however. We show here that T cell-specific deletion of the gene encoding TAK1 resulted in reduced development of thymocytes, especially of regulatory T cells expressing the transcription factor Foxp3. In mature thymocytes, TAK1 was required for interleukin 7-mediated survival and T cell receptor-dependent activation of transcription factor NF-kappaB and the kinase Jnk. In effector T cells, TAK1 was dispensable for T cell receptor-dependent NF-kappaB activation and cytokine production, but was important for proliferation and activation of the kinase p38 in response to interleukins 2, 7 and 15. Thus, TAK1 is essential for the integration of T cell receptor and cytokine signals to regulate the development, survival and function of T cells.