both immunologists and physicians who wish to know more about this fascinating and important host defence system. The introductory chapter by John Weiler presents a historical background to research on the complement system and describes the biochemical events occurring during activation of the system. In Chapter 2 Alastair Dodds and Tony Day discuss the phylogeny and evolution of the complement system. The techniques of protein chemistry and molecular biology have provided powerful insights into the modular structure of complement proteins and the evolution of the complement system. The structure and organisation of the complement genes is described in Chapter 3 by Ken Reid and Duncan Campbell. This chapter describes the modular structure of the complement proteins and some of the mutations which are responsible for deficiencies of individual components.