The complement (C) system in man and its relationship to disease has been the subject of intensive research. In this review, we update the information concerning the nature of the various C components, and present some of the similarities between structure and function of the C components and their respective genes. The clinical problems which are encountered in individuals with acquired C abnormalities and with a genetically determined deficiency of a single component provide helpful clues to understanding the affected patients and the possible functional importance of the particular deficient component. The steady progress in identifying both normal variants of C components and the gene defects which produce C deficiencies offers the prospect of correlating structure of the C components with possible pathogenic roles in disease. Genetically determined C abnormalities are more commonly recognized during childhood. An appreciation of the basic aspects of the C system is a helpful tool for the pediatric nephrologist.