Recent advances in the structural analysis of the genes and proteins for immunoglobulin Fc domain receptors have provided a molecular characterization of this complex family. The wide cellular distribution of these receptors and their functional heterogeneity are reflected in the diversity of molecules which bind antibody and immune complexes. The detailed analysis of the IgG and IgE Fc receptors has indicated that these molecules have evolved from a common precursor through gene duplication. Similarities among these receptors, in both structure and function have emerged. Thus, the Fc receptors provide an example of a class of molecules in which conserved domains are combined with divergent sequences to yield a diversity of function.