This chapter has attempted to provide an overview of the chemistry and biology of the eight cellular complement receptors. A summary of the information presented is shown in Table 3. Special emphasis was given to new developments in the field since three other reviews on the subject have appeared previously [34, 92,93]. A number of recent findings have significantly enhanced our understanding of these receptors. Investigations concerned with CRI continue to provide new insights into its biochemistry and maintain CRI as the best characterized complement receptor. The polypeptide chain composition and immunochemistry of CR2 and CR3 have just been established and their ligands clearly defined. The recent association of a CR3 deficiency and susceptibility to bacterial infections established the biologic relevance of at least one of the complement receptors. Future studies will be needed to complete the biochemical definition of the complement receptors and establish their structure-function relationships. Currently, only three of the receptors (C1q receptor, CR1, and C5a receptor) have been characterized with respect to ligand-binding affinity and only three (CR1, CR2, and CR3) have been isolated and their polypeptide chain compositions established. More work is needed to elucidate the biologic responses that are elicited upon receptor engagement and to determine the relative significance of these responses under physiologic conditions. Very little is currently known about the molecular biology of the various receptors or how receptor expression can be regulated by humoral factors. This information will most likely become available in the next few years and will clarify the role of complement receptors in cellular regulation and immune reactions.