Viral infections are first detected by a set of innate immunity receptors that detect primary infections by pathogens, and trigger a transcriptional response. Among the induced target genes, type I interferons (IFNs) are central to the antiviral response of the host. The receptors and signaling pathways that mediate the strong induction of the synthesis of these cytokines have long remained elusive. In the past few years, Toll-like receptors (TLRs) emerged as important sensors of infections. Several TLRs participate in the recognition of virus infection, interacting in particular with viral nucleic acids. Upon activation, TLRs interact with different cytosolic adapter molecules and activate transcription factors of the nuclear factor-kappaB and IFN regulatory factor families that concur to mediate induction of IFN-alpha/beta and other inflammatory cytokines. In addition to the transmembrane TLRs, cytosolic helicases also detect viral nucleic acids, and trigger type I IFN synthesis.