The complement system has long been recognized as an important component of host immunity. The proteins of the complement cascade are known to participate in several physiologic and pathophysiologic processes relevant to the development and progression of bacterial invasion. A brief overview of complement activation and the inhibitors of the cascade is provided. Some of the investigations that have enhanced understanding of the antiinfective role of complement and the pathophysiologic consequences of unregulated complement activation in advanced bacterial sepsis are summarized. New areas of research that could lead to novel strategies for preventing and treating bacterial sepsis in children are explored. They include exploiting the role of complement in the immune response to foreign antigens for the development of vaccines, the use of cytokines to enhance the synthesis and secretion of complement components, and the use of complement inhibitors to prevent complications of advanced bacterial sepsis. Copyright © 2001 by W.B. Saunders Company