The protein kinase TAK1 mediates the activation of NF-κB in response to stimulation by proinflammatory cytokines and microbial pathogens in the innate immunity pathways. However, the physiological function of TAK1 in the adaptive immunity pathways is unclear. By engineering mice lacking TAK1 in T cells, here, we show that TAK1 is essential for thymocyte development and activation in vivo. Deletion of TAK1 prevented the maturation of single-positive thymocytes displaying CD4 or CD8, leading to reduction of T cells in the peripheral tissues. Thymocytes lacking TAK1 failed to activate NF-κB and JNK and were prone to apoptosis upon stimulation. Our results provide the genetic evidence that TAK1 is required for the activation of NF-κB in thymocytes and suggest that TAK1 plays a central role in both innate and adaptive immunity. © 2006 by The National Academy of Sciences of the USA.