The type-III interferons (IFNs) are the most recently discovered IFNs in the human immune system and have important, but as yet poorly characterized, functions in innate and adaptive immunity that complement their antiviral functions. It is now becoming clear that these type-III IFNs have a functional niche where epithelial surfaces interact with the adaptive immune system, that their antiviral capability is not as highly developed as that of the type-I IFNs, and that they have their own profile of immunomodulatory functions; specifically, they are key modulators of the T-helper (Th)2 response.