This chapter focuses on studies of the murine and human leukocyte FcγR and FCεR I, with particular reference to the structural characterization of these receptors, the molecular nature of their interaction with immunoglobulin (Ig), and their mechanisms of signal transduction. In addition, the chapter also reviews different aspects of FcμR, FcαR, the poly Ig receptor, the receptor for the transport of Ig in neonatal gut, and receptors for IgD. The characterization of the proteins, transcripts, and genes of the different classes of both these FcR families— FcγR and FCεR I—has largely been completed and those for IgM, IgD, and IgA receptors still to be completed. Attention is now turning to understanding the nature of the mechanisms by which these receptors and their subunits are able to mediate their functions. The structural analysis of the functional basis of FcγR and FCεR signaling is beginning to shed some light on the way these receptors trigger biological responses, and the molecular dissection of the pathways involved in signaling is also underway. The understanding of these interactions may provide the means to devise strategies to inhibit pathophysiological effects of FcR function that would have far reaching implications in the treatment of antibody induced hypersensitivity.