An important contribution to preventing social insecurity can be made by strong social security programmes that promote collective as well as personal security: in particular, they act as built-in automatic stabilizers with social and political as well as economic benefits for the whole society. By contrast, weaker systems have a destabilizing effect built in that reduces their preventive effectiveness. The ways in which social security systems can contribute to preventing social and economic insecurity have become less effective in many countries. Reasons for the relative neglect of prevention are considered together with the changes that have weakened systems' preventive elements specifically. The structural context within which social security policies have to work, particularly the quality and quantity of employment as well as the interaction with other public programmes, has of course a major effect on their ability to prevent. There is still much that can be achieved by schemes with closer attention directed to what is needed to prevent insecurity. Proposals to strengthen the preventive contribution in social security programmes are offered to promote greater debate of these issues and to restore the goal of prevention as a central objective.