The last ten years have seen an explosive growth in our understanding of IFNI" The cloning of the cDNAs for IFNI'and its receptor have made available large amounts of highly purified recombinant IFNI'and soluble IFNI'receptor. In addition, highly specific neutralizing monoclonal anti bodies have been generated to both of these proteins. Using these reagents, IFNI'and the IFNI'receptor have been characterized on a molecular basis. Structure-function studies carried out on these proteins have identified key molecular regions that are required for biologic activity. Moreover, a great deal is now known concerning the physiologic role that IFNy plays in vivo. In this review we focus on the new developments in the areas ofIFNy biochemistry and biology and pay particular attention to the IFNI'receptor and three aspects of IFNI'biology that are of special interest to immun ologists: host defense, inflammation, and autoimmunity.