The term complement was introduced more than 100 years ago to refer to a group of plasma factors important in host defense and in the destruction of microorganisms. We now know that there are 3 separate activation pathways that appeared at different times in evolution: the classical, alternative, and lectin pathways. Two of these appear before the evolution of the adaptive immune system and do not require antibody for initiation. All pathways come together to activate C3, the principle opsonic protein of the complement cascade, and all continue together to the generation of biologically active factors, such as C5a, and to lysis of cells and microbes. In general, complete deficiencies of complement proteins are rare, although partial or complete deficiencies of one of the proteins that initiates the lectin pathway, mannose-binding lectin, are far more common. Although genetically controlled complement defects are rare, defects in the proteins in the circulation and on cell membranes that downregulate complement so as to limit uncontrolled inflammation are more common. A number of these are discussed, and because new methods of treatment are currently being introduced, one of these defects, CI inhibitor deficiency associated with hereditary angioedema, is discussed in some detail.