Interferons (IFNs) can serve as the first line of immune defense against viral infection. The identification of IFN-λs 1, 2, 3 & 4 (termed as type III IFNs) has revealed that the antiviral immune response to viruses contains more components than the type I IFNs that have been known for more than 50 years. IFN-λs are IFN-λ1 (IL-29), IFN-λ2 (IL-28a), IFN-λ3 (IL-28b) and IFN-λ4, which resembles IFN-λ3. IFN-λs have type I-IFN-like immune responses and biological activities, but our knowledge of these novel players in the antiviral response is not well established. In this review, we try to describe the current information on the expression and function of IFN-λs in the innate antiviral immune defense and IFN-λ2’s role in regulating and shaping the adaptive immune response. We suggest that IFN-λs are key antiviral cytokines, directly performing an antiviral immune response at epithelial surfaces in the early stages of viral infection, and that these cytokines also skew the balance of Th1 and Th2 cells to Th1 phenotype. In addition, genetic polymorphisms in IFN-λ genes can impair antiviral immune responses in clinical treatment.