Signal transduction through cell surface receptors such as those specific for antigen, lymphokines or immunoglobulin (Ig) is an important event for many facets of cell growth and differentiation. The group of receptors which interact with the Fc region of Ig are involved in a number of immune functions, including phagocytosis, antibody dependent cellular cytotoxicity and release of inflammatory mediators (see¹ for review). The Fc receptors that interact with immunoglobulin E (IgE) have been divided into two predominant receptors. Fc _∈ RI, or the high affinity receptor for IgE, is found predominantly on mast cells and basophils and is involved in cytokine and allergic mediator release from these cells^l. Discovered somewhat later, the Fc _∈ RII, or the low affinity receptor for IgE, is expressed on a wide variety of hematopoietic cell types. The finding that the Fc _∈ RII was synonymous with the B cell differentiation antigen CD23^2,3 led to a merging of two areas of investigation for this protein; the study of the role of the Fc _∈ RII/CD23 (the names are used interchangeably) in B cell activation/differentiation and the study of the role of the Fc _∈ RII in IgE production. This review chapter will concentrate on summarizing recent developments regarding the Fc _∈ RII with regard to both the structure and function of this molecule. Several other recent reviews on this subject are also available^4–6.