During the last several years, the mechanism of IFNγ-dependent signal transduction has been the focus of intense investigation. This research has recently culminated in the elucidation of a comprehensive molecular understanding of the events that underlie IFNγ-induced cellular responses. The structure and function of the IFNγ receptor have been defined. The mechanism of IFNγ signal transduction has been largely elucidated, and the physiologic relevance of this process validated. Most recently, the molecular events that link receptor ligation to signal transduction have been established. Together these insights have produced a model of IFNγ signaling that is nearly complete and that serves as a paradigm for signaling by other members of the cytokine receptor superfamily.