IRAK-4 is a protein kinase that is pivotal in mediating signals for innate immune responses. Here, we report that IRAK-4 signaling is also essential for eliciting adaptive immune responses. Thus, in the absence of IRAK-4, in vivo T cell responses were significantly impaired. Upon T cell receptor stimulation, IRAK-4 is recruited to T cell lipid rafts, where it induces downstream signals, including protein kinase Cθ activation through the association with Zap70. This signaling pathway was found to be required for optimal activation of nuclear factor κB. Our findings suggest that T cells use this critical regulator of innate immunity for the development of acquired immunity, suggesting that IRAK-4 may be involved in direct signal cross talk between the two systems.