Gamma interferon (IFN-γ) is an important cytokine in the host defense against infection by viral and microbial pathogens. IFN-γ induces a variety of physiologically significant responses that contribute to immunity. Treatment of animal cells with IFN-γ or infection with viral or microbial pathogens leads to changes in the level of expression of several target genes as revealed by DNA microarray analyses. The signaling pathways leading to the induction of IFN-γ-regulated gene products and, in some cases, their biochemical functions have been defined in exquisite detail. Studies of transgenic mutant mice deficient in proteins of the IFN-γ response pathway firmly establish the importance of IFN-γ in immunity.