The aim of this paper is not to demonstrate the effectiveness of seat belts, but to discuss in detail some of the factors which compromise their optimal performance and to describe the nature and frequencies of serious injuries sustained by belt users. Many authors (1-6) have shown that the seat belt provides considerable protection for the user in the accident situation, but its efficiency is nevertheless highly dependent on the maintenance of the integrity of the passenger compartment, on the correct use of the restraint system and on the characteristics of the system. By analysing those files compiled by the Accident Research Unit at Birmingham University in which a restrained occupant of a car was seriously injured, some of the features resulting in limited protection have been identified.