The transcription factors interferon regulatory factor 3 (IRF3) and NF-kappaB are required for the expression of many genes involved in the innate immune response. Viral infection, or the binding of double-stranded RNA to Toll-like receptor 3, results in the coordinate activation of IRF3 and NF-kappaB. Activation of IRF3 requires signal-dependent phosphorylation, but little is known about the signaling pathway or kinases involved. Here we report that the noncanonical IkappaB kinase homologs, IkappaB kinase-epsilon (IKKepsilon) and TANK-binding kinase-1 (TBK1), which were previously implicated in NF-kappaB activation, are also essential components of the IRF3 signaling pathway. Thus, IKKepsilon and TBK1 have a pivotal role in coordinating the activation of IRF3 and NF-kappaB in the innate immune response.