tIn recent years, complement analysis of body fluids and biopsies, going far beyond C3 and C4, has signifi-cantly enhanced our understanding of the disease process. Such expanded complement analysis allows fora more precise differential diagnosis and for critical monitoring of complement-targeted therapy. Thesechanges are a result of the growing understanding of the involvement of complement in a diverse set ofdisorders. To appreciate the importance of proper complement analysis, it is important to understandthe role it plays in disease. Historically, it was the absence of complement as manifested in severe infec-tion that was noted. Since then complement has been connected to a variety of inflammatory disorders,such as autoimmune diseases and hereditary angioedema. While the role of complement in the rejectionof renal grafts has been known longer, the significant impact of complement. In certain nephropathieshas now led to the reclassification of some rare kidney diseases and an increased role for complementanalysis in diagnosis. Even more unexpected is that complement has also been implicated in neural, oph-talmological and dermatological disorders. With this level of involvement in some varied and impactfulhealth issues proper complement testing is clearly important; however, analysis of the complementsystem varies widely among laboratories. Except for a few proteins, such as C3 and C4, there are nei-ther well-characterized standard preparations nor calibrated assays available. This is especially true forthe inter-laboratory variation of tests which assess classical, alternative, or lectin pathway function. Inaddition, there is a need for the standardization of the measurement of complement activation prod-ucts that are so critical in determining whether clinically relevant complement activation has occurredin vivo. Finally, autoantibodies to complement proteins (e.g. anti-C1q), C3 and C4 convertases (C3 andC4 nephritic factor) or to regulatory proteins (e.g. anti-C1inhibitor, anti-factor H) are important in defin-ing autoimmune processes and diseases based on complement dysregulation. To improve the quality ofcomplement laboratory analysis a standardization commmittee of the International Complement Society(ICS) and the International Union of Immunological Societies (IUIS) was formed to provide guidelines formodern complement analysis and standards for the development of international testing programs.