This review describes the evolution of our knowledge of the transmission of immunoglobulin G (IgG) from mother to infant and the factors which regulate the persistence of IgG in the circulation. These apparently unrelated processes involve the same Fc receptor, FcRn (n = neonatal). FcRn appears to carry out these diverse roles by binding to IgG and then either transporting the bound IgG across cells (transcytosis) or recycling its cargo back to the cell surface (control of catabolism). IgG that is taken up by cells in the absence of binding to FcRn undergoes degradation. Thus, FcRn is the "protective" receptor that serves to maintain IgG homeostasis and deliver IgGs across cellular barriers.