The half-life of the two most abundant proteins in blood, immunoglobulin G (IgG) and serum albumin, is extraordinary (approximately 19-23 days) compared to other circulating proteins. This phenomenon secures a broad biodistribution throughout the body of both molecules. The long half-life has made IgG the natural choice for engineering of antibody based therapeutics, while albumin is used as a fusion partner for or carrier of drugs. Remarkably, the half-life of these unrelated proteins has been shown prolonged by a receptor recycling pathway mediated by a common cell bound receptor named the neonatal Fc receptor (FcRn). This review summarizes our current understanding of FcRn function and discusses its relevance for development of new IgG and albumin based therapeutics and diagnostics.