The complement system consists of about two dozen plasma and cell membrane proteins which function as cofactors in defense against pathogenic microbes and in the generation of many immunopathogenic disorders. The complexity of this system and its role in other biological functions has been appreciated within the last two decades. Recognition of genetic deficiencies of the complement proteins and their phenotypic expression has provided additional insights into the physiological role of the complement system. Complementary DNA (cDNA) clones for most of the complement components are now available, and the gene structures for many have been elucidated. Application of molecular biological methods to studies of the complement system and its deficiencies has permitted both the determination of primary structure and chromosomal localization of complement genes (Figure 1) and the capacity to elucidate the molecular basis of complement deficiency disorders.