Since 1993 the Safety-Critical Systems Club has hosted the Safety-critical Systems Symposium (SSS) each February. Every year the programme has reflected what is then of particular interest to the safety community - in ways of working, in analysis techniques, in technology, in standards, and in research work that is on the point of being useful to practitioners. This book presents the papers delivered at the landmark fifteenth SSS. A subject that is mostly neglected by safety practitioners but which, if studied more carefully, could lead to both technical and economic efficiencies, as well as more effective protection, is the relationship between safety and security. The management of both is based on risk analysis and there are indications that the analyses could effectively be combined. The Symposium has offered papers on this subject in the past, and this year there are three. Continuing the trend of recent years, there are five papers on the development of safety cases, which are intended to demonstrate, or claim, the achievement of safety - in defined applications and under given circumstances. Some of the papers report on experiences in the field, but others venture to propose new ways in which safety cases may be used or extended.