The complement system is a group of proteins which plays a major role in the processing and removal of microorganisms and tissue breakdown products from the circulation and extracellular spaces. The system is activated by a wide range of targets, and activation leads to the production of opsonins, chemotaxis of granulocytes, cell lysis and other biological activities. Inappropriate overactivation of the system contributes to inflammatory tissue damage in the host, while inadequate activation leads to accumulation of immune complexes and other debris in the circulation, and susceptibility to infection. The biology and biochemistry of the system is now adequately understood, and attempts can be made to manipulate the activation and activities of the system for potential therapeutic purposes. The reviews in this volume summarise what is known of the ways in which the complement system can be activated, by interaction with antibodies, microorganisms, cell debris, and complex carbohydrates and how the activities and activation of the system have been modified, accidentally or by design, in vitro or in vivo by drugs, venoms, particulate carbohydrates, specific antibodies, synthetic peptides and other reagents.