Although the last few decades have produced considerable technical literature on safety belts, the question of whether they really work continues to be a recurrent issue of public interest. For example, a number of Canada's mandatory belt wearing laws were challenged in 1992, based on an unpublished study claiming that belt-wearing caused more harm than it prevented. The present paper summarizes the technical evidence for the effectiveness of safety belts, and of laws requiring their use. It is hoped that this short paper, by synthesizing and summarizing currently available information, will be a useful document to have available when future challenges arise. The evidence regarding belts is discussed under the following three broad headings: 1.Impact biomechanics -- the mechanical basis of injury causation. 2.The observed effect of safety belts, given that a crash has occurred. 3.Changes in casualties after the passage of mandatory wearing laws.