Interest in complement developed at the end of the nineteenth century from observations on cellular and humoral defense mechanisms against bacteria. It was recognized at that time that there were factors in body fluids of animals and man that were capable of killing and lysing bacteria in the absence of cellular factors. Due to the efforts of two of the founders of immunology, Bordet and Ehrlich, and their colleagues, by 1912 the multicomponent nature of complement action was well recognized, the sequence of reaction of the components in the lysis of erythrocytes was defined, complement fixation as a major tool for studying antibody-antigen interaction was well established, and studies on the physicochemical properties of the components had been started. Yet, with a few notable exceptions, research on complement was largely abandoned by most "mainstream" immunologists for the following two or three decades. When one looks at the contents of the present volume, it is hard to imagine that as recently as 20 years ago, there were probably fewer than ten major laboratories where complement research was the primary theme. The contents attest to the fact that there are today dozens of laboratories on three continents where research on complement is pursued in depth. It is not easy to point to all the advances that have occurred in complement research during the past few years.