Minimal performance requirements for seat belts with which motor vehicles are equipped were introduced in Switzerland in 1961 by SKASG. However, usage, age, or shock may adversely affect the protection potential of a seat bell. Such a deterioration is not covered by performance standards for new, unused restraint systems. In order to investigate the conditions under which an old belt or a belt which has been exposed to a heavy shock in a car crash has to be replaced multiple dynamic and static tests were performed with different belt webbings. Moreover, the mechanical characteristics of belts which had been in use for several years were measured. It was found that repeated shock or age does not significantly change the mechanical properties of the belt webbings which were examined. Unless there is a visible damage in a restraint system such as abrasion of the webbing or malfunction of a buckle or retractor, an old belt or a belt which has been used in a car crash does not necessarily have to be replaced as a rule.