The paper briefly characterizes the complement system and gives the results of experimental and clinical studies of complement components in preeclampsia. The functions of the complement system, its activation mechanisms, complement activation pathways (classical, alternative, and lectin), and the role of complement components are set forth. It is stated that the activation of the complement system is associated with the development of pregnancy complications and inhibition of complement activation reduces the manifestations of preeclampsia. It is pointed that there are scarce data on the role of the complement system in pregnant women in general and in those with preeclampsia in particular. Emphasis is laid on the need for further investigations of the complement system in pregnant women.