This chapter deals with interferon γ (IFNγ), focusing on its structure. IFNγ is a pleotropic cytokine that induces antiviral, antiproliferative, and immunomodulatory effects on numerous target cells. Natural forms of human IFNγ are composed of two 143-amino-acid peptide chains that are posttranslationally modified to contain an N-terminal pyroglutamic acid residue, N-linked glycosylation at two positions, and a heterogeneous C terminus containing the positively charged sequence KTGKRKR (residues 125-131). The crystal structure of human IFNγ has revealed the tight association of two peptide chains (composed of six α-helices, labeled A to F, from the N to C terminus) into a remarkable intertwined helix topology to form a symmetric dimer. In agreement with solution and cell-surface binding studies, the structure of the IFNγ/IFNγR1 complex has revealed the symmetric binding of two IFNγR1s to one IFNγ dimer. © 2010 Elsevier Inc. All rights reserved.