The interferon (IFN) regulatory factors (IRF) consist of a growing family of related transcription proteins first identified as regulators of the IFN-alpha/beta gene promoters, as well as the IFN-stimulated response element (ISRE) of some IFN-stimulated genes. IRF-3 was originally identified as a member of the IRF family based on homology with other IRF family members and on binding to the ISRE of the IFN-stimulated gene 15 (ISG15) promoter. Several recent studies have focused attention on the unique molecular properties of IRF-3 and its role in the regulation of IFN gene expression. IRF-3 is expressed constitutively in a variety of tissues, and the relative levels of IRF-3 mRNA do not change in virus-infected or IFN-treated cells. Following virus infection, IRF-3 is posttranslationally modified by protein phosphorylation at multiple serine and threonine residues, located in the carboxy-terminus of IRF-3. Phosphorylation causes the cytoplasmic to nuclear translocation of IRF-3, stimulation of DNA binding, and increased transcriptional activation, mediated through the association of IRF-3 with the CBP/p300 coactivator. The purpose of this review is to summarize recent investigations demonstrating the important role of IRF-3 in cytokine gene transcription. These studies provide the framework for a model in which virus-dependent phosphorylation of IRF-3 alters protein conformation to permit nuclear translocation, association with transcriptional partners, and primary activation of IFN and IFN-responsive genes.